[RISCV] Select signed and unsigned bitfield extracts for XTHeadBb

The XTHeadBb extension hab both signed and unsigned bitfield
extraction instructions (TH.EXT and TH.EXTU, respectively) which have
previously only been supported for sign extension on byte, halfword,
and word-boundaries.

This adds the infrastructure to use TH.EXT and TH.EXTU for arbitrary
bitfield extraction.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D144229