[X86] Merge ADDSUB/SUBADD detection into single methods that can detect either and...

[X86] Merge ADDSUB/SUBADD detection into single methods that can detect either and indicate what they found.

Previously, we called the same functions twice with a bool flag determining whether we should look for ADDSUB or SUBADD. It would be more efficient to run the code once and detect either pattern with a flag to tell which type it found.

Differential Revision: https://reviews.llvm.org/D44540

llvm-svn: 327730