[llvm] [RISCV] Introduce VLOptimizer pass (PR #108640)

[Philip Reames via llvm-commits]

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+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py UTC_ARGS: --version 2
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preames wrote:

These three test files seem to have valuable coverage regardless of which approach we take towards reducing VL.  Can you commit these separately and rebase?

https://github.com/llvm/llvm-project/pull/108640