[RISCV] Improve vector fceil/ffloor lowering by changing FRM.

[RISCV] Improve vector fceil/ffloor lowering by changing FRM.

This adds new VFCVT pseudoinstructions that take a rounding mode operand. A custom inserter is used to insert additional instructions to change FRM around the
VFCVT.

Some of this is borrowed from D122860, but takes a somewhat different direction. We may migrate to that patch, but for now I was trying to keep this as independent from
RVV intrinsics as I could.

A followup patch will use this approach for FROUND too.

Still need to fix the cost model.

Reviewed By: arcbbb

Differential Revision: https://reviews.llvm.org/D133238