[RISCV] Custom legalize vp.merge for mask vectors. (#120479) The default legalization uses vmslt with a vector of XLen to compute a mask. This doesn't work if the type isn't legal. For fixed vectors it will scalarize. For scalable vectors it crashes the compiler. This patch uses an alternate strategy that promotes the i1 vector to an i8 vector and does the merge. I don't claim this to be the best lowering. I wrote it quickly almost 3 years ago when a crash was reported in our downstream. Fixes #120405.
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