[RISCV] Model vxrm control for vsmul, vssra, vssrl, vnclip, and vnclipu

Depends on D151397.

This patch follows the patch-set of D151395. This patch seeks to
update all the remaining fixed-point intrinsics to model vxrm
control, adding rounding mode control for `vsmul`, `vssra`,
`vssrl`, `vnclip`, and `vnclipu`.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D152879