[AArch64][GISel] Optimize bitreverse intrinsic for vector types

[madhur13490]

2 x s32, 2 x i64, 4 x s16, 4 x s32, 8 x s16 types are going through suboptimal lowering. Looking at SDAG, revXX followed by rbit is more optimal.
godbolt: https://godbolt.org/z/nP8jssKh8

[llvmbot]

@llvm/issue-subscribers-backend-aarch64

Author: Madhur Amilkanthwar (madhur13490)

2 x s32, 2 x i64, 4 x s16, 4 x s32, 8 x s16 types are going through suboptimal lowering. Looking at SDAG, `revXX` followed by `rbit` is more optimal. godbolt: https://godbolt.org/z/nP8jssKh8