[RISCV] Combine FP_TO_INT to vfwcvt/fvncvt

Adds new pseudo instructions to make sure that the fcvt instructions
have all rounding mode (RM) and unsigned (XU) variants across
single-width, widening and narrowing conversions.
And likewise, extends the VL patterns to accompany them. We don't add
new VL nodes for the widening/narrowing conversions though, instead we
just add specific patterns for vfcvts on those wider/narrower types.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D142102