rb_str_buf_append: add a fast path for ENC_CODERANGE_VALID

If the RHS has valid encoding, and both strings have the same
encoding, we can use the fast path.

However we need to update the LHS coderange.

```
compare-ruby: ruby 3.2.0dev (2022-07-21T14:46:32Z master cdbb9b8) [arm64-darwin21]
built-ruby: ruby 3.2.0dev (2022-07-21T15:08:55Z string-concat-vali.. 61c61c564a) [arm64-darwin21]
warming up...

|                    |compare-ruby|built-ruby|
|:-------------------|-----------:|---------:|
|binary_concat_7bit  |    550.089k|  568.002k|
|                    |           -|     1.03x|
|utf8_concat_7bit    |    552.799k|  563.395k|
|                    |           -|     1.02x|
|utf8_concat_UTF8    |    329.423k|  410.754k|
|                    |           -|     1.25x|
```

benchmark/string_concat.yml

@@ -1,9 +1,9 @@
 prelude: |
   CHUNK = "a" * 64
-  BCHUNK = "a".b * 64
+  UCHUNK = "€" * 32
   GC.disable # GC causes a lot of variance
 benchmark:
-  binary_concat_utf8: |
+  binary_concat_7bit: |
     buffer = String.new(capacity: 4096, encoding: Encoding::BINARY)
     buffer << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK
     buffer << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK
@@ -13,17 +13,7 @@ benchmark:
     buffer << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK
     buffer << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK
     buffer << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK
-  binary_concat_binary: |
-    buffer = String.new(capacity: 4096, encoding: Encoding::BINARY)
-    buffer << BCHUNK << BCHUNK << BCHUNK << BCHUNK << BCHUNK << BCHUNK << BCHUNK << BCHUNK
-    buffer << BCHUNK << BCHUNK << BCHUNK << BCHUNK << BCHUNK << BCHUNK << BCHUNK << BCHUNK
-    buffer << BCHUNK << BCHUNK << BCHUNK << BCHUNK << BCHUNK << BCHUNK << BCHUNK << BCHUNK
-    buffer << BCHUNK << BCHUNK << BCHUNK << BCHUNK << BCHUNK << BCHUNK << BCHUNK << BCHUNK
-    buffer << BCHUNK << BCHUNK << BCHUNK << BCHUNK << BCHUNK << BCHUNK << BCHUNK << BCHUNK
-    buffer << BCHUNK << BCHUNK << BCHUNK << BCHUNK << BCHUNK << BCHUNK << BCHUNK << BCHUNK
-    buffer << BCHUNK << BCHUNK << BCHUNK << BCHUNK << BCHUNK << BCHUNK << BCHUNK << BCHUNK
-    buffer << BCHUNK << BCHUNK << BCHUNK << BCHUNK << BCHUNK << BCHUNK << BCHUNK
-  utf8_concat_utf8: |
+  utf8_concat_7bit: |
     buffer = String.new(capacity: 4096, encoding: Encoding::UTF_8)
     buffer << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK
     buffer << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK
@@ -33,3 +23,13 @@ benchmark:
     buffer << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK
     buffer << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK
     buffer << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK
+  utf8_concat_UTF8: |
+    buffer = String.new(capacity: 4096, encoding: Encoding::UTF_8)
+    buffer << UCHUNK << UCHUNK << UCHUNK << UCHUNK << UCHUNK << UCHUNK << UCHUNK << UCHUNK
+    buffer << UCHUNK << UCHUNK << UCHUNK << UCHUNK << UCHUNK << UCHUNK << UCHUNK << UCHUNK
+    buffer << UCHUNK << UCHUNK << UCHUNK << UCHUNK << UCHUNK << UCHUNK << UCHUNK << UCHUNK
+    buffer << UCHUNK << UCHUNK << UCHUNK << UCHUNK << UCHUNK << UCHUNK << UCHUNK << UCHUNK
+    buffer << UCHUNK << UCHUNK << UCHUNK << UCHUNK << UCHUNK << UCHUNK << UCHUNK << UCHUNK
+    buffer << UCHUNK << UCHUNK << UCHUNK << UCHUNK << UCHUNK << UCHUNK << UCHUNK << UCHUNK
+    buffer << UCHUNK << UCHUNK << UCHUNK << UCHUNK << UCHUNK << UCHUNK << UCHUNK << UCHUNK
+    buffer << UCHUNK << UCHUNK << UCHUNK << UCHUNK << UCHUNK << UCHUNK << UCHUNK

string.c

@@ -3329,9 +3329,22 @@ VALUE
 rb_str_buf_append(VALUE str, VALUE str2)
 {
     int str2_cr = rb_enc_str_coderange(str2);
-    if (str2_cr == ENC_CODERANGE_7BIT && str_enc_fastpath(str)) {
-        str_buf_cat4(str, RSTRING_PTR(str2), RSTRING_LEN(str2), true);
-        return str;
+
+    if (str_enc_fastpath(str)) {
+        switch (str2_cr) {
+          case ENC_CODERANGE_7BIT:
+            str_buf_cat4(str, RSTRING_PTR(str2), RSTRING_LEN(str2), true);
+            return str;
+          case ENC_CODERANGE_VALID:
+            if (ENCODING_GET_INLINED(str) == ENCODING_GET_INLINED(str2)) {
+                str_buf_cat4(str, RSTRING_PTR(str2), RSTRING_LEN(str2), true);
+                int str_cr = ENC_CODERANGE(str);
+                if (UNLIKELY(str_cr != ENC_CODERANGE_VALID)) {
+                    ENC_CODERANGE_SET(str, RB_ENC_CODERANGE_AND(str_cr, str2_cr));
+                }
+                return str;
+            }
+        }
     }
 
     rb_enc_cr_str_buf_cat(str, RSTRING_PTR(str2), RSTRING_LEN(str2),