[PowerPC] Handle inline assembly clobber of link regsiter

This patch adds the handling of clobbers of the link register LR for inline
assembly.

This patch is to fix:
https://bugs.llvm.org/show_bug.cgi?id=50147

Reviewed By: nemanjai, #powerpc

Differential Revision: https://reviews.llvm.org/D101657

(cherry picked from commit 15051f0b)