Allow basefee as Yul identifier for EVMVersion < london

This was done to prevent basefee from being a breaking change. This change will be removed in 0.9.0.

TODO revert this commit in breaking.

test/libsolidity/syntaxTests/types/magic_block_basefee_error.sol

@@ -2,14 +2,8 @@ contract C {
     function f() public view returns (uint) {
         return block.basefee;
     }
-    function g() public view returns (uint ret) {
-        assembly {
-            ret := basefee()
-        }
-    }
 }
 // ====
-// EVMVersion: =berlin
+// EVMVersion: <=berlin
 // ----
 // TypeError 5921: (74-87): "basefee" is not supported by the VM version.
-// TypeError 5430: (183-190): The "basefee" instruction is only available for London-compatible VMs (you are currently compiling for "berlin").