[AArch64] Emit zext move when the source of the zext is AssertZext or AssertSext

When the source of the zext is AssertZext or AssertSext, it is hard to know any information about the upper 32 bits,
so we should insert a zext move before emitting SUBREG_TO_REG to define the lower 32 bits.

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D87771