[PowerPC][HTM] Fix impossible reg-to-reg copy assert with ttest builtin
This is exposed by our internal testing.
The reduced testcase will assert with "Impossible reg-to-reg copy"
We can't use COPY to do 32-bit to 64-bit conversion.
Reviewers: kbarton, hfinkel, nemanjai
Reviewed By: hfinkel
Subscribers: hiraditya, MaskRay, llvm-commits
Differential Revision: https://reviews.llvm.org/D64499