AMDGPU: Clean up maximum3/minimum3 tests (#93025)

These were using patterns copied from older tests, before non-kernel
functions were supported and manually written checks. Also stop using
-flat-for-global, which only exists to try to share tests between SI/CI
and VI+.

This was also missing test coverage, we're incorrectly forming
maximum3/minimum3 pre-gfx12. This is a pre-commit before fixing that.

Author: arsenm