[AArch64] Emit AssertZExt for i1 arguments

AAPCS requires i1 argument to be zero-extended to 8-bits by the
caller. Emit a new AArch64ISD::ASSERT_ZEXT_BOOL hint (or AssertZExt
for GlobalISel) to enable some optimization opportunities. In
particular, when the argument is forwarded to the callee, we can avoid
zero-extension and use it as-is.

Differential Revision: https://reviews.llvm.org/D107160