[dev.unified] cmd/compile/internal/noder: implicit conversions for writer.assignStmt

[dev.unified] cmd/compile/internal/noder: implicit conversions for writer.assignStmt

This CL inserts implicit conversions for simple N:N assignment
statements within Unified IR. A subsequent CL will handle N:1
assignments.

Change-Id: I7e204c6ee9ffdb9fa2bc9146315fd79735c04628
Reviewed-on: https://go-review.googlesource.com/c/go/+/413516
Reviewed-by: David Chase <drchase@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>