[AArch64][SVE2] Rename bitperm feature to sve2-bitperm

Summary:
The bitperm feature flag is now prefixed with SVE2, as it is for all other SVE2
extensions

Patch by Maciej Gabka.

Reviewers: sdesmalen, rovka, chill, SjoerdMeijer, rengolin

Reviewed By: SjoerdMeijer, rengolin

Differential Revision: https://reviews.llvm.org/D65327

llvm-svn: 367124