AMDGPU: Clean up maximum3/minimum3 tests

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