[RISCV] Don't iterate SEW set again for floating-point

We have already iterated over `FPList`, so there is no need to get
and loop over SEW set again. Or we will generate some useless pseudo
instructions like `PseudoXXX_VF16_M1_E64`.

We add SEW to FPR_Info so that we can get it later. And we rewrite
FPR_Info to reduce arguments incidentally.

This should reduce the size of files generated by TableGen.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D152524