Make early if conversion dependent upon the subtarget and add

Make early if conversion dependent upon the subtarget and add
a subtarget hook to enable. Unconditionally add to the pass pipeline
for targets that might want to use it. No functional change.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@209340 91177308-0d34-0410-b5e6-96231b3b80d8