Tests for basefee as identifier in inline assembly

Also added basefee to `EVMVersion::hasOpcode(...)`

liblangutil/EVMVersion.cpp

@@ -46,8 +46,9 @@ bool EVMVersion::hasOpcode(Instruction _opcode) const
 		return hasChainID();
 	case Instruction::SELFBALANCE:
 		return hasSelfBalance();
+	case Instruction::BASEFEE:
+		return hasBaseFee();
 	default:
 		return true;
 	}
 }
-

test/libsolidity/semanticTests/inlineAssembly/basefee_berlin_function.sol

@@ -0,0 +1,21 @@
+contract C {
+    function f() public view returns (uint ret) {
+        assembly {
+            let basefee := sload(0)
+            ret := basefee
+        }
+    }
+    function g() public pure returns (uint ret) {
+        assembly {
+            function basefee() -> r {
+                r := 1000
+            }
+            ret := basefee()
+        }
+    }
+}
+// ====
+// EVMVersion: =berlin
+// ----
+// f() -> 0
+// g() -> 1000

test/libsolidity/syntaxTests/inlineAssembly/basefee_reserved_london.sol

@@ -0,0 +1,12 @@
+contract C {
+    function f() public view returns (uint ret) {
+        assembly {
+            let basefee := sload(0)
+            ret := basefee
+        }
+    }
+}
+// ====
+// EVMVersion: =london
+// ----
+// ParserError 5568: (98-105): Cannot use builtin function name "basefee" as identifier name.