[AArch64][LoadStoreOptimizer] Ignore undef registers when checking rename...

 [AArch64][LoadStoreOptimizer] Ignore undef registers when checking rename register used between paired instructions.

The content of undef registers are not used in meaningful ways, when checking
if a rename register is used between paired instructions we should ignore
undef registers.

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D119305