[RISCV] Fold vmerge into op with undef passthru by using vmerge's vl (#179862)
Currently we only fold an op into vmerge if we know the smaller of the
two vls, because we can't increase the vl.
However if the op's passthru is undef, then we can just use vmerge's vl
because the lanes past op's vl were undef anyway. We need to make sure
that the op doesn't access memory though. Other instructions where the
result depends on the VL should already be handled by the
RISCVII::elementsDependOnVL check below.
This is probably always profitable because even though we increase vl,
we remove a vmerge which needs to process vl elements anyway.
This removes some regressions in #179622.
llvm-project
63918f51 - [RISCV] Fold vmerge into op with undef passthru by using vmerge's vl (#179862)
