[PowerPC] fix a bug in redundant compare elimination

This patch fixes a bug in the redundant compare elimination reported in https://reviews.llvm.org/rL320786 and re-enables the optimization.

The redundant compare elimination assumes that we can replace signed comparison with unsigned comparison for the equality check. But due to the difference in the sign extension behavior we cannot change the opcode if the comparison is against an immediate and the most significant bit of the immediate is one.

Differential Revision: https://reviews.llvm.org/D41385

llvm-svn: 321147