cmd/compile: improve rotations for riscv64

cmd/compile: improve rotations for riscv64

Enable canRotate for riscv64, enable rotation intrinsics and provide
better rewrite implementations for rotations. By avoiding Lsh*x64
and Rsh*Ux64 we can produce better code, especially for 32 and 64
bit rotations. By enabling canRotate we also benefit from the generic
rotation rewrite rules.

Benchmark on a StarFive VisionFive 2:

               │   rotate.1   │              rotate.2               │
               │    sec/op    │   sec/op     vs base                │
RotateLeft-4     14.700n ± 0%   8.016n ± 0%  -45.47% (p=0.000 n=10)
RotateLeft8-4     14.70n ± 0%   10.69n ± 0%  -27.28% (p=0.000 n=10)
RotateLeft16-4    14.70n ± 0%   12.02n ± 0%  -18.23% (p=0.000 n=10)
RotateLeft32-4   13.360n ± 0%   8.016n ± 0%  -40.00% (p=0.000 n=10)
RotateLeft64-4   13.360n ± 0%   8.016n ± 0%  -40.00% (p=0.000 n=10)
geomean           14.15n        9.208n       -34.92%

Change-Id: I1a2036fdc57cf88ebb6617eb8d92e1d187e183b2
Reviewed-on: https://go-review.googlesource.com/c/go/+/560315
Reviewed-by: M Zhuo <mengzhuo1203@gmail.com>
Run-TryBot: Joel Sing <joel@sing.id.au>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Mark Ryan <markdryan@rivosinc.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: David Chase <drchase@google.com>