[RISCV] Pre-process FP SPLAT_VECTOR to RISCVISD::VFMV_V_F_VL

This patch builds on top of D119197 to canonicalize floating-point
SPLAT_VECTOR as RISCVISD::VFMV_V_F_VL as a pre-process ISel step.

This primarily benefits scalable-vector VP code, where our VP patterns
only match VFMV_V_F_VL to reduce the burden on our ISel patterns, but
where at the same time, scalable-vector code doesn't custom-legalize
SPLAT_VECTOR.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D117670