[RISCV] Implement straight-forward bf16<->int conversion cases

This ports over the test cases half-convert.ll and implements patterns
or RISCVISelLowering.cpp changes for all of the most straight-forward
cases (those that don't require changes outside of lib/Target/RISCV).
The remaining cases and noted poor codegen for saturating conversions
will be handled in follow-up patches.

Differential Revision: https://reviews.llvm.org/D156943