[AArch64][ShrinkWrap] Fix bug in prolog clobbering live reg when shrink wrapping.

Summary: See bug https://llvm.org/bugs/show_bug.cgi?id=26642

Reviewers: qcolombet, t.p.northover

Subscribers: aemerson, rengolin, mcrosier, llvm-commits

Differential Revision: http://reviews.llvm.org/D17350

llvm-svn: 261349