[SelectionDAG][RISCV] Don't unroll 0/1-type bool VSELECTs

This patch extends the cases in which the legalizer is able to express
VSELECT in terms of XOR/AND/OR. When dealing with a VSELECT between
boolean vector types, the mask itself is an all-ones or all-ones value
of the operand type, so a 0/1 boolean type behaves identically to a 0/-1
type.

This greatly helps RISC-V which relies on expansion for these nodes. It
also allows scalable-vector bool VSELECTs to use the default expansion,
where before it would crash in SelectionDAG::UnrollVectorOp.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D103147