[PowerPC][HTM] Fix impossible reg-to-reg copy assert with ttest builtin

Summary:
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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D64499

llvm-svn: 366255