[RISCV][VP] Add basic RVV codegen for vp.fcmp

This patch adds the necessary infrastructure to lower vp.fcmp via
ISD::VP_SETCC to RVV instructions.

Most notably this patch adds cond-code legalization for VP_SETCC,
reusing the existing TargetLowering::LegalizeSetCCCondCode by passing in
additional SDValue parameters for the Mask and EVL. This method then
uses VP operations to legalize the condcode.

There is still a general lack of canonicalization on VP_SETCC as opposed
to SETCC which results in worse code than is theoretically possible.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D123051