[3/8][RISCV] Add rounding mode control variant for vfmul, vfdiv, vfrdiv, vfwmul

Depends on D154629

For the cover letter of the patch-set, please checkout D154628.

This is the 3rd patch of the patch-set.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D154631