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

[Luke Lau via llvm-commits]

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lukel97 wrote:

Here's a test case for the issue:
```
---
name: copy_multiple_use
body: |
  bb.0:
    liveins: $x8, $v0, $v8
    %avl:gprnox0 = COPY $x8
    %passthru:vrnov0 = COPY $v8
    %x:vrnov0 = PseudoVLE32_V_M1 $noreg, $noreg, %avl, 5 /* e32 */, 2 /* tu, ma */ :: (load unknown-size)
    %copy1:vrnov0 = COPY %x
    %mask:vmv0 = COPY $v0
    PseudoVSE8_V_M1 %copy1, $noreg, %avl, 5 /* e8 */
    %z:vrnov0 = PseudoVMERGE_VVM_M1 %passthru, %passthru, %copy1, %mask, %avl, 5 /* e32 */
```

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