Minor perf improvements to quantized mat mul shader.

[trivedivivek]

Summary: The diff includes minor performance improvements to the quantized matrix multiplication shader.

Differential Revision: D84998542

[pytorch-bot]

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🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/15261

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❌ 7 New Failures, 2 Pending, 2 Unrelated Failures

As of commit 3002776 with merge base 0dbf06e ():

NEW FAILURES - The following jobs have failed:

• pull / test-samsung-models-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 56cfdd8271ed0e28e222d9426a0178aa2036e0d9830fa4477208cbee0bb1cf86 /exec failed with exit code 1
• pull / unittest-nxp-neutron / linux-job (gh)
  RuntimeError: Command docker exec -t 008ffc1604cba310ba8351b36f2010b4606e6e1631dd97445bdedfe619ff0bc5 /exec failed with exit code 1
• Test CUDA Builds / export-gemma3-cuda-non-quantized / linux-job (gh)
  RuntimeError: Command docker exec -t 85d58b285767181321c2c915a108770108add481aaf44e0e1ad331ad73f26846 /exec failed with exit code 2
• Test CUDA Builds / export-voxtral-cuda-non-quantized / linux-job (gh)
  RuntimeError: Command docker exec -t 8b54b7ef9c6a8dc5affc8bc198803bbffc56f6a958c7f6890a9c294c56425b89 /exec failed with exit code 2
• Test CUDA Builds / export-voxtral-cuda-quantized-int4-tile-packed / linux-job (gh)
  RuntimeError: Command docker exec -t 01c8fba73338dceabd1f63c04eb4838f69ba1eb9891572ef3c93c11e95b246d2 /exec failed with exit code 2
• Test CUDA Builds / export-voxtral-cuda-quantized-int4-weight-only / linux-job (gh)
  RuntimeError: Command docker exec -t 36fcf96aa6f79ce08491d708b793c7229419701918801050fe467adf610c10ad /exec failed with exit code 2
• Test Metal Backend / export-voxtral-metal-artifact / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 2

BROKEN TRUNK - The following jobs failed but were present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• pull / unittest / linux / linux-job (gh) (trunk failure)
• pull / unittest-editable / linux / linux-job (gh) (trunk failure)

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[meta-codesync]

@trivedivivek has exported this pull request. If you are a Meta employee, you can view the originating Diff in D84998542.