[X86] Reduce memory allocations in X86TargetMachine::getSubtargetImpl

We performed a number of memory allocations each time getTTI was called,
remove them by using SmallString.
No functionality change intended.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@270246 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed