constify TargetMachine parameter for X86TargetLowering.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218804 91177308-0d34-0410-b5e6-96231b3b80d8
Rework the PPC TargetMachine so that the non-function specific
overrides happen at TargetMachine creation and not on every
subtarget creation.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218805 91177308-0d34-0410-b5e6-96231b3b80d8