[llvm] [RISCV] Clear kill flags for FalseReg in foldVMergeToMask (PR #170438)

[Luke Lau via llvm-commits]

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lukel97 wrote:

I'm just realising now we need to restrict lookThruCopies to only look through COPYs with one non-debug use. I think we only need to do this for the True operand because we're modifying the True operand, the False and Passthru operands shouldn't need the restriction. Maybe we can add an argument to lookThruCopies like `bool OneUseOnly`?

I presume you're planning on cherry-picking this so maybe we should combine that fix in this PR too?

https://github.com/llvm/llvm-project/pull/170438