[CK_TILE] Fix FMHA BWD IGLP incorrect results due to AGPR
misallocation (#5991)
## Motivation
After PR #5790 removed the `if constexpr(FmhaMask::IsMasking)` guard
around the
`num_total_loop <= 0` early-exit check, the IGLP pipeline
(`BlockFmhaBwdDQDKDVPipelineKRKTRVRIGLP`) produces incorrect dK/dV
gradients for
non-masking kernels (even with fix in #5915). Assembly inspection
confirms that the CFG change causes the LLVM
register allocator to reuse AGPR accumulators as scratch destinations in
the dK/dV
reduction loop, breaking the loop-carried accumulation across Q-tile
iterations.
## Technical Details
- Add `[[unlikely]]` to the `num_total_loop <= 0` early-exit in
`BlockFmhaBwdDQDKDVPipelineKRKTRVRIGLP`. This attribute is load-bearing:
it
restores the CFG shape that the register allocator needs to correctly
assign
dedicated AGPRs to each column of the dK/dV accumulator.
- Only the IGLP pipeline is affected; the other two BWD pipelines do not
exhibit
this issue.
## Test Plan
## Test Result
## Submission Checklist
- [x] Look over the contributing guidelines at
https://github.com/ROCm/ROCm/blob/develop/CONTRIBUTING.md#pull-requests.
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