cmd/compile: ensure dictionary assignment statements are defining statements

The problem in 51355 is that escape analysis decided that the
dictionary variable was captured by reference instead of by value. We
want dictionaries to always be captured by value.

Escape analysis was confused because it saw what it thought was a
reassignment of the dictionary variable. In fact, it was the only
assignment, it just wasn't marked as the defining assignment. Fix
that.

Add an assert to make sure this stays true.

Fixes #51355

Change-Id: Ifd9342455fa107b113f5ff521a94cdbf1b8a7733
Reviewed-on: https://go-review.googlesource.com/c/go/+/388115


Trust: Keith Randall <khr@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
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Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Dan Scales <danscales@google.com>
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