[PowerPC] don't check CTR clobber in hardware loop insertion pass

We added a new post-isel CTRLoop pass in D122125. That pass will expand
the hardware loop related intrinsic to CTR loop or normal loop based
on the loop context. So we don't need to conservatively check the CTR
clobber now on the IR level.

Reviewed By: lkail

Differential Revision: https://reviews.llvm.org/D135847