[RISCV] Generalize performFP_TO_INTCombine to vectors

Like in the scalar domain, combine calls to (fp_to_int (ftrunc X)) on
scalable and fixed-length vectors into a single vfcvt instruction.
For truncating rounds, the static vfcvt.rtz rounding mode is used.
Otherwise use the VFCVT_RM_ variants to set the rounding mode
dynamically.
Closes https://github.com/llvm/llvm-project/issues/56737

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D141599