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

Currently, many attempts to lower loads and stores on buffer fat
pointers lower directly to intrnsic 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.

Author: krzysz00