[GlobalISel][X86] refactoring X86InstructionSelector.cpp .NFC.
[GlobalISel][X86] refactoring X86InstructionSelector.cpp .NFC.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@313484 91177308-0d34-0410-b5e6-96231b3b80d8
(cherry picked from commit 414ee511a79b162a310a940615d5458a67acbff8)

apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm