cmd/compile: add rule for post-decomposed growslice optimization
The recently added rule only works before decomposing slices.
Add a rule that works after decomposing slices.
The reason we need the latter is because although the length may
be a constant, it can be hidden inside a slice that is not constant
(its pointer or capacity might be changing). By applying this
optimization after decomposing slices, we can find more cases
where it applies.
Fixes #56440
Change-Id: I0094e59eee3065ab4d210defdda8227a6e897420
Reviewed-on: https://go-review.googlesource.com/c/go/+/446277
Run-TryBot: Keith Randall <khr@golang.org> Reviewed-by: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org> Reviewed-by: Keith Randall <khr@google.com>