[x86] avoid FileCheck assert duplication with retl/retq regex; NFC

This was suggested in PR35003:
https://bugs.llvm.org/show_bug.cgi?id=35003

32-bit checks may be identical to 64-bit (if we avoid those pesky scalar params!).

I'll check in the script change shortly assuming this doesn't anger any bots.

llvm-svn: 316223