[NFC][AMDGPU] Pre-commit tests for buffer contents legalization (llvm…

…#110559)

Currently, many attempts to lower loads and stores on buffer fat
pointers lower directly to intrinsic calls that will be unsupported by
or crash codegen (ex, storing a [2 x i32], a <6 x half>, or an i160).

Record the current behavior to make the effects of the fix more visible
in an upcoming PR.