[X86] Don't disable slow INC/DEC if optimizing for size

Summary:
Just because INC/DEC is a little slow on some processors doesn't mean we shouldn't prefer it when optimizing for size.

This appears to match gcc behavior.

Reviewers: chandlerc, zvi, RKSimon, spatel

Reviewed By: RKSimon

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D37177

llvm-svn: 312866