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.
Really minor but I find TAFRM quite hard to parse. I dunno if TA_FRM is preferable?