[XCore] Add instruction pattern for bitrev

Summary:
Add support for lowering bitreverse to the bitrev instruction.
Fix https://bugs.llvm.org/show_bug.cgi?id=34628.

Reviewers: RKSimon, rtrieu, robertlytton

Reviewed By: RKSimon

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D74748