[LegalizeVectorOps] Use all ones mask when expanding i1 VP_SELECT.

We were previously using the condition as the mask. By the semantics
of VP operations, that means that anywhere the condition is false
returns poison and not the false operand.

Use an all ones mask instead.

No tests are affected because RISC-V drops the mask when lowering.

Reviewed By: fakepaper56

Differential Revision: https://reviews.llvm.org/D149310