AMDGPU: Ensure flat loads are broken into dword in functions

We were assuming we could rely on the flat scratch init detection
to imply if there are possible flat addressed stack objects, which
doesn't work outside of a kernel. We should have a way to prove
if a given flat access can't access the stack.

We could use a not-stack parameter attribute to avoid
these splits.

Make the minimally correct change for GlobalISel; I'll address
this better in my larger patch to rewrite load and store legalization.

Fixes: SWDEV-218237