Fix 14127 js from int to int casting (nim-lang#15918)

* fix nim-lang#14127 from int to int casting

* add test for nim-lang#14127

* use template for test, also test uint2int

* move to tests/types/t14127_cast_number.nim targets:c cpp js

compiler/jsgen.nim

@@ -2406,8 +2406,7 @@ proc genCast(p: PProc, n: PNode, r: var TCompRes) =
     r.res = "($1 $2)" % [r.res, trimmer]
   elif toInt:
     if fromInt:
-      let trimmer = unsignedTrimmer(dest.size)
-      r.res = "($1 $2)" % [r.res, trimmer]
+      return
     elif fromUint:
       if src.size == 4 and dest.size == 4:
         # XXX prevent multi evaluations

tests/types/t14127_cast_number.nim

@@ -0,0 +1,30 @@
+discard """
+  targets: "c cpp js"
+"""
+block: # bug #14127
+  template int2uint(T) =
+    var a = -1
+    let b = cast[T](a)
+    doAssert b < 0
+    let c = b + 1
+    doAssert c is T
+    doAssert c == 0
+
+  int2uint(int8)
+  int2uint(int16)
+  int2uint(int32)
+  int2uint(int64)
+
+block: # maybe related
+  template uint2int(T) =
+    var a = 3
+    let b = cast[T](a)
+    doAssert b > 0
+    let c = b - 1
+    doAssert c is T
+    doAssert c == 2
+
+  uint2int(uint8)
+  uint2int(uint16)
+  uint2int(uint32)
+  uint2int(uint64)