[AMDGPU] Split fp min/max atomics test. NFC.

Split out f32 buffer, f64 buffer and image atomics. This just makes
it easier to test subtargets that only have some of these
instructions.

Differential Revision: https://reviews.llvm.org/D120407

GitOrigin-RevId: e66b1b73854e43f7e822fbb3be9e9875e6780761
4 files changed