[dev.unified] cmd/compile/internal/noder: refactor stmtAssign generation

[dev.unified] cmd/compile/internal/noder: refactor stmtAssign generation

Eliminate some code duplication between assignment statements and
variable declarations, so they're easier to extend with implicit
conversions.

Change-Id: I605cf7817e3cb230f2c4612b777d8023c926e8b2
Reviewed-on: https://go-review.googlesource.com/c/go/+/413515
Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: David Chase <drchase@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>