[dev.regabi] cmd/compile: adjust one case in walkexpr

The mid-case n := n.(*ir.AssignExpr) does not lend itself
well to pulling the code into a new function, because n will
be a function argument and will not be redeclarable.

Change-Id: I673f2aa37eea64b083725326ed3fa36447bcc7af
Reviewed-on: https://go-review.googlesource.com/c/go/+/279426


Trust: Russ Cox <rsc@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>