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[RISCV] Add matching of codegen patterns to RISCV Bit Manipulation Zbb asm…

Authored by lewis-revill on Jul 15 2020, 3:50 AM.

Description

[RISCV] Add matching of codegen patterns to RISCV Bit Manipulation Zbb asm instructions

This patch provides optimization of bit manipulation operations by
enabling the +experimental-b target feature.
It adds matching of single block patterns of instructions to specific
bit-manip instructions from the base subset (zbb subextension) of the
experimental B extension of RISC-V.
It adds also the correspondent codegen tests.

This patch is based on Claire Wolf's proposal for the bit manipulation
extension of RISCV:
https://github.com/riscv/riscv-bitmanip/blob/master/bitmanip-0.92.pdf

Differential Revision: https://reviews.llvm.org/D79870