[TableGen] Use StringRef::compare instead of != and <. NFC.

The previous code would always do 1 or 2 prefix compares;
explicitly only do one.

This speeds up debug -gen-asm-matcher by ~10% (e.g. X86: 40s -> 35s).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@273583 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed