[RISCV] Add inline expansion for vector fround.

This avoids a crash for scalable vectors and or scalarization for
fixed vectors.

The algorithm is different enough that I don't think it makes sense
to merge with ceil/floor/trunc. Algorithm is adapted from gcc's X86
SSE2 output.

Reviewed By: frasercrmck

Differential Revision: https://reviews.llvm.org/D117247