[RISCV] Lower splats of non-constant i1s as SETCCs

[RISCV] Lower splats of non-constant i1s as SETCCs

This patch adds support for splatting i1 types to fixed-length or
scalable vector types. It does so by lowering the operation to a SETCC
of the equivalent i8 type.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D101465