AArch64/GlobalISel: Narrow stack passed argument access size

This fixes a verifier error in the testcase from bug 47619.

The stack passed s3 value was widened to 4-bytes, and producing a
4-byte memory access with a < 1 byte result type. We need to either
widen the result type or narrow the access size. This copies the code
directly from the AMDGPU handling, which narrows the load size. I
don't like that every target has to handle this, but this is currently
broken on the 11 release branch and this is the simplest fix.

This reverts commit 42bfa7c6.