[llvm] [AMDGPU] Add additional coverage tests for llvm.amdgcn.tanh, NFC (PR #202864)

[Changpeng Fang via llvm-commits]

changpeng wrote:

Fixed the broken RUN line and regenerated test expectations:

1. **Fixed broken RUN line**: Removed the empty prefix in line 7's check-prefixes (was `,GFX13,GFX13-GISEL-REAL16`, now `GFX13,GFX13-GISEL-REAL16`)
2. **Regenerated test checks**: Updated CHECK lines to match current llc output, which changes `s_wait_bvhcnt` to `s_wait_rtscnt` for GFX13

The test file already had the correct function signature structure (regular C functions for vector tests, `amdgpu_ps` for scalar/constant tests), so no changes were needed there.

Ready for review!

https://github.com/llvm/llvm-project/pull/202864