[dev.regabi] cmd/compile: fix map assignment order

After the previous cleanup/optimization CLs, ascompatee now correctly
handles map assignments too. So remove the code from order.mapAssign,
which causes us to assign to the map at the wrong point during
execution. It's not every day you get to fix an issue by only removing
code.

Thanks to Cuong Manh Le for test cases and continually following up on
this issue.

Passes toolstash -cmp. (Apparently the standard library never uses
tricky map assignments. Go figure.)

Fixes #23017.

Change-Id: Ie0728103d59d884d00c1c050251290a2a46150f9
Reviewed-on: https://go-review.googlesource.com/c/go/+/281172


Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>