[dev.regabi] cmd/compile: bind closure vars during SSA constructions

For function literals that aren't inlined or directly called, we need
to pass their arguments via a closure struct. This also means we need
to rewrite uses of closure variables to access from this closure
struct.

Currently we do this rewrite in a pass before walking begins. This CL
moves the code to SSA construction instead, alongside binding other
input parameters.

Change-Id: I13538ef3394e2d6f75d5b7b2d0adbb00db812dc2
Reviewed-on: https://go-review.googlesource.com/c/go/+/281352


Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Keith Randall <khr@golang.org>