[RISCV] Add DAG combine to fold (fp_to_int_sat (ffloor X)) -> (select X == nan, 0, (fcvt X, rdn))

Similar for ceil, trunc, round, and roundeven. This allows us to use
static rounding modes to avoid a libcall.

This is similar to D116771, but for the saturating conversions.

This optimization is done for AArch64 as isel patterns.
RISCV doesn't have instructions for ceil/floor/trunc/round/roundeven
so the operations don't stick around until isel to enable a pattern
match. Thus I've implemented a DAG combine.

I'm only handling saturating to i64 or i32. This could be extended
to other sizes in the future.

Reviewed By: asb

Differential Revision: https://reviews.llvm.org/D116864