[RISCV][CodeGen] Add codegen pattern for FLI instruction in experimental zfa extension

This patch implements experimental support for the RISCV Zfa extension as specified here: https://github.com/riscv/riscv-isa-manual/releases/download/draft-20221119-5234c63/riscv-spec.pdf, Ch. 25. This extension has not been ratified. Once ratified, it'll move out of experimental status.

This change adds codegen support for load-immediate instructions (fli.s/fli.d/fli.h).

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D141560