[AArch64][GlobalISel] Emit extloads for ZExt/SExt values in assignValueToAddress

When a value is expected to be extended, we should emit an extended load rather
than a normal G_LOAD.

Add checklines to arm64-abi.ll which show that we now emit the correct loads.

For ease of comparison: https://godbolt.org/z/8WvY6EfdE

Differential Revision: https://reviews.llvm.org/D107313