[llvm] [RISCV] Reduce the VL of both operands in VMERGE_VVM (PR #144759)

[Philip Reames via llvm-commits]

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@@ -112,7 +112,7 @@ bool RISCVVectorPeephole::tryToReduceVL(MachineInstr &MI) const {
   //
   // TODO: We can handle a bunch more instructions here, and probably
   // recurse backwards through operands too.
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preames wrote:

Yeah, this TODO is definitely stale.  Longer term, we should probably eliminate this bit of code in favor of VLOptimizer just to have fewer copies doing roughly the same thing.

https://github.com/llvm/llvm-project/pull/144759