cmd/compile: use shlx&shrx instruction for GOAMD64>=v3

cmd/compile: use shlx&shrx instruction for GOAMD64>=v3

The SHRX/SHLX instruction can take any general register as the shift count operand, and can read source from memory. This CL introduces some operators to combine load and shift to one instruction.

For #47120

Change-Id: I13b48f53c7d30067a72eb2c8382242045dead36a
Reviewed-on: https://go-review.googlesource.com/c/go/+/385174
Reviewed-by: Keith Randall <khr@golang.org>
Trust: Cherry Mui <cherryyz@google.com>