[RISCV] Support vector conversions between fp and i1

This patch custom lowers FP_TO_[US]INT and [US]INT_TO_FP conversions
between floating-point and boolean vectors. As the default action is
scalarization, this patch both supports scalable-vector conversions and
improves the code generation for fixed-length vectors.

The lowering for these conversions can piggy-back on the existing
lowering, which lowers the operations to a supported narrowing/widening
conversion and then either an extension or truncation.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D103312