[RISCV] Lower RVV vector SELECTs to VSELECTs

This patch optimizes the code generation of vector-type SELECTs (LLVM
select instructions with scalar conditions) by custom-lowering to
VSELECTs (LLVM select instructions with vector conditions) by splatting
the condition to a vector. This avoids the default expansion path which
would either introduce control flow or fully scalarize.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D104772