[PowerPC] Update `incr` after resetting the register in MI

After performing signed extension, we update the register in MI. We should also update `incr` register which is tracking the register in `MI`.

Fixes https://github.com/llvm/llvm-project/issues/61882.

Reviewed By: #powerpc, shchenz

Differential Revision: https://reviews.llvm.org/D147594