[X86] Move getX86ConditionCode() from X86FastISel.cpp to X86InstrInfo.cpp. NFC

Summary:
Move getX86ConditionCode() from X86FastISel.cpp to X86InstrInfo.cpp so it can be used by GloabalIsel instruction selector.
This is a pre-commit for a patch I'm working on to support G_ICMP. NFC.

Reviewers: zvi, guyblank, delena

Reviewed By: guyblank, delena

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D33038

llvm-svn: 302767