Remove rotate generation from walk.  Remove OLROT and ssa.Lrot* opcodes.
Generate rotates during SSA lowering for architectures that have them.

This CL will allow rotates to be generated in more situations,
like when the shift values are determined to be constant
only after some analysis.

Fixes #18254

Change-Id: I8d6d684ff5ce2511aceaddfda98b908007851079
Reviewed-on: https://go-review.googlesource.com/34232
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>