[NV] WaveReadLaneAt.Index test failing on NV 50-series

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```
FAIL: OffloadTest-d3d12 :: WaveOps/WaveReadLaneAt.index.test (260 of 262)
******************** TEST 'OffloadTest-d3d12 :: WaveOps/WaveReadLaneAt.index.test' FAILED ********************
Exit Code: 1

Command Output (stdout):
--
# RUN: at line 121
c:\actions-runner\_work\offload-test-suite\offload-test-suite\llvm-project\build\bin\split-file.exe C:\actions-runner\_work\offload-test-suite\offload-test-suite\OffloadTest\test\WaveOps\WaveReadLaneAt.index.test C:\actions-runner\_work\offload-test-suite\offload-test-suite\llvm-project\build\tools\OffloadTest\test\d3d12\WaveOps\Output\WaveReadLaneAt.index.test.tmp
# executed command: 'c:\actions-runner\_work\offload-test-suite\offload-test-suite\llvm-project\build\bin\split-file.exe' 'C:\actions-runner\_work\offload-test-suite\offload-test-suite\OffloadTest\test\WaveOps\WaveReadLaneAt.index.test' 'C:\actions-runner\_work\offload-test-suite\offload-test-suite\llvm-project\build\tools\OffloadTest\test\d3d12\WaveOps\Output\WaveReadLaneAt.index.test.tmp'
# RUN: at line 122
"C:\actions-runner\_work\offload-test-suite\offload-test-suite\DXC\build\bin\dxc.exe" -T cs_6_5 -Fo C:\actions-runner\_work\offload-test-suite\offload-test-suite\llvm-project\build\tools\OffloadTest\test\d3d12\WaveOps\Output\WaveReadLaneAt.index.test.tmp.o C:\actions-runner\_work\offload-test-suite\offload-test-suite\llvm-project\build\tools\OffloadTest\test\d3d12\WaveOps\Output\WaveReadLaneAt.index.test.tmp/source.hlsl
# executed command: 'C:\actions-runner\_work\offload-test-suite\offload-test-suite\DXC\build\bin\dxc.exe' -T cs_6_5 -Fo 'C:\actions-runner\_work\offload-test-suite\offload-test-suite\llvm-project\build\tools\OffloadTest\test\d3d12\WaveOps\Output\WaveReadLaneAt.index.test.tmp.o' 'C:\actions-runner\_work\offload-test-suite\offload-test-suite\llvm-project\build\tools\OffloadTest\test\d3d12\WaveOps\Output\WaveReadLaneAt.index.test.tmp/source.hlsl'
# RUN: at line 123
c:\actions-runner\_work\offload-test-suite\offload-test-suite\llvm-project\build\bin\offloader.exe -debug-layer C:\actions-runner\_work\offload-test-suite\offload-test-suite\llvm-project\build\tools\OffloadTest\test\d3d12\WaveOps\Output\WaveReadLaneAt.index.test.tmp/pipeline.yaml C:\actions-runner\_work\offload-test-suite\offload-test-suite\llvm-project\build\tools\OffloadTest\test\d3d12\WaveOps\Output\WaveReadLaneAt.index.test.tmp.o
# executed command: 'c:\actions-runner\_work\offload-test-suite\offload-test-suite\llvm-project\build\bin\offloader.exe' -debug-layer 'C:\actions-runner\_work\offload-test-suite\offload-test-suite\llvm-project\build\tools\OffloadTest\test\d3d12\WaveOps\Output\WaveReadLaneAt.index.test.tmp/pipeline.yaml' 'C:\actions-runner\_work\offload-test-suite\offload-test-suite\llvm-project\build\tools\OffloadTest\test\d3d12\WaveOps\Output\WaveReadLaneAt.index.test.tmp.o'
# .---command stdout------------
# | Using DirectX API
# | Configuring execution on device: NVIDIA GeForce RTX 5070
# | RootSignature created.
# | Descriptor heap created.
# | Command structures created.
# | Creating SRV: { Size = 16, Register = t0, Space = 0 }
# | Creating UAV: { Size = 16, Register = u1, Space = 0, HasCounter = 0 }
# | Creating UAV: { Size = 16, Register = u2, Space = 0, HasCounter = 0 }
# | Creating UAV: { Size = 16, Register = u3, Space = 0, HasCounter = 0 }
# | EltSize = 4 NumElts = 4
# | SRV: HeapIdx = 0 EltSize = 4 NumElts = 4
# | EltSize = 4 NumElts = 4
# | UAV: HeapIdx = 1 EltSize = 4 NumElts = 4 HasCounter = 0
# | EltSize = 4 NumElts = 4
# | UAV: HeapIdx = 2 EltSize = 4 NumElts = 4 HasCounter = 0
# | EltSize = 4 NumElts = 4
# | UAV: HeapIdx = 3 EltSize = 4 NumElts = 4 HasCounter = 0
# | Buffers created.
# | Event prepared.
# | PSO created.
# | Compute command list created.
# | Compute commands executed.
# | Read data back.
# `-----------------------------
# .---command stderr------------
# | Test failed: TestMix
# | Comparison Rule: BufferExact
# | Expected:
# | ---
# | Name: ExpectedOutMix
# | Format: Int32
# | Stride: 4
# | Data: [ 2, 3, 1, 0 ]
# | OutputProps:
# | Height: 0
# | Width: 0
# | Depth: 0
# | ...
# | Got:
# | ---
# | Name: OutMix
# | Format: Int32
# | Stride: 4
# | Data: [ 0, 0, 0, 0 ]
# | OutputProps:
# | Height: 0
# | Width: 0
# | Depth: 0
# | ...
# | Full Hex 64bit representation of Expected Buffer Values:
# | [ 0x2, 0x3, 0x1, 0x0 ]
# | Full Hex 64bit representation of Actual Buffer Values:
# | [ 0x0, 0x0, 0x0, 0x0 ]
# |
# `-----------------------------
# error: command failed with exit status: 1
```

Start with OffloadTest/test/WaveOps/WaveReadLaneAt.index.test and reproduce the dxc.exe/offloader.exe run on the NVIDIA GeForce RTX 5070 shown in the report. Investigate the TestMix mismatch, where ExpectedOutMix is [2, 3, 1, 0] but OutMix is [0, 0, 0, 0]; done means the test passes with the expected buffer output.

Written by the indexing model from the issue text.