[AArch64] Fix Copy Elemination for negative values

Redundant Copy Elimination was eliminating a MOVi32imm -1 when it
determined that the value of the destination register is already -1.
However, it didn't take into account that the MOVi32imm zeroes the upper
32 bits (which are FFFFFFFF) and therefore cannot be eliminated.

Reviewed By: paulwalker-arm

Differential Revision: https://reviews.llvm.org/D93100