[RISCV] Don't use constantpool for floating-point value if the value can be...

[RISCV] Don't use constantpool for floating-point value if the value can be easily constructed by integer sequence and a floating-point move.

In addition, this commit does the following combine

vfmv.v.f + fmv.[dhw].x -> vmv.v.x
vfmv.s.f + fmv.[dhw].x -> vmv.s.x
vfmerge.vfm + fmv.[dhw].x -> vmerge.vxm

Differential Revision: https://reviews.llvm.org/D142953