cmd/compile: split up the addressing mode on OpAMD64CMP*loadidx* always

Benchmarking suggests that the combo instruction is notably slower,
at least in the places where we measure.

Updates #37955

Change-Id: I829f1975dd6edf38163128ba51d84604055512f4
Reviewed-on: https://go-review.googlesource.com/c/go/+/228157


Run-TryBot: David Chase <drchase@google.com>
Reviewed-by: Keith Randall <khr@golang.org>