builtins: Fix up ICE in inline_string_cmp [PR109258]

The PR109086 r13-6690 inline_string_cmp change to
      if (diff != result)
        emit_move_insn (result, diff);
regressed
FAIL: go.test/test/fixedbugs/bug207.go,  -O2 -g  (internal compiler error: in emit_move_insn, at expr.cc:4224)
The problem is the Go FE doesn't mark __builtin_memcmp as pure (I'll also
send patch for that) and so result is const0_rtx when the call lost its lhs
and the above move ICEs because moving something into const0_rtx is obviously
invalid.
I think it is better not to rely on all FEs having these *cmp functions
pure anD DCE being performed.  The following patch just punts from the
inline expansion in that case, so we just emit normal library call.

2023-03-24  Jakub Jelinek  <jakub@redhat.com>

	PR middle-end/109258
	* builtins.cc (inline_expand_builtin_bytecmp): Return NULL_RTX early
	if target == const0_rtx.