AMDGPU: Implement early ifcvt target hooks.

AMDGPU: Implement early ifcvt target hooks.

Leave early ifcvt disabled for now since there are some
shader-db regressions.

This causes some immediate improvements, but could be better.
The cost checking that the pass does is based on critical path
length for out of order CPUs which we do not want so it skips out
on many cases we want.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@293016 91177308-0d34-0410-b5e6-96231b3b80d8