[llvm] [AMDGPU] Track DS prefetch flush points on single-path loop. (PR #227274)

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@llvm/pr-subscribers-backend-amdgpu

Author: Vikash Gupta (vg0204)

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<summary>Changes</summary>

On `GFX12+`, the DS prefetch preheader flush tracked flush points only in a single-block loop. Walk the loop in issue order when each block has one in-loop successor, and use that order for the same tracking. A loop with an in-loop branch is unchanged.

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Full diff: https://github.com/llvm/llvm-project/pull/227274.diff


2 Files Affected:

- (modified) llvm/lib/Target/AMDGPU/SIInsertWaitcnts.cpp (+46-12) 
- (added) llvm/test/CodeGen/AMDGPU/waitcnt-loop-ds-prefetch-multiblock.mir (+98) 


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https://github.com/llvm/llvm-project/pull/227274