[RISCV] Support vector strict_fsetcc/fsetccs.

The patch supports vector strict_fsetcc/fsetccs. Instead of revserving fflags,
the method to implement scalar quiet compares, the patch implement quiet
compares by masking the signaling compares when either input is NaN [0].

[0]: https://github.com/riscv/riscv-v-spec/blob/master/v-spec.adoc#vector-floating-point-compare-instructions

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D147998