[llvm] [RISCV] Check the legality of source vector types in matchSplatAsGather (PR #133028)

[Luke Lau via llvm-commits]

https://github.com/lukel97 commented:

I thought matchSplatAsGather was only supposed to be run with all types legal. Maybe the best place to fix this would be in performDAGCombine?

https://github.com/llvm/llvm-project/blob/5d0c18e489f9919342911d2443d1847deb3d3204/llvm/lib/Target/RISCV/RISCVISelLowering.cpp#L19133-L19142

Can we replace the `!isTypeLegal(VT)` with `DCI.isBeforeLegalize()`? I presume that would also cover the source type being illegal.

https://github.com/llvm/llvm-project/pull/133028