Miscompile after D155071

[topperc]

I have a tail undefined unmasked vrgather.vi with VL=2 followed by a tail undefined vmerge with VL=4. This gets combined to a tail agnostic masked undisturbed masked vrgather.vi with VL =2. I think this needs to be a tail undisturbed VL to account for elements 2 and 3 from the vmerge becoming part of the tail?

I don't have a reduced test case

cc: @preames

[llvmbot]

@llvm/issue-subscribers-backend-risc-v

[lukel97]

Candidate patch: https://reviews.llvm.org/D158161

[lukel97]

/cherry-pick 6e532f9 007b41b

[llvmbot]

/branch llvm/llvm-project-release-prs/issue64754

[llvmbot]

/pull-request llvm/llvm-project-release-prs#628