This is a re-attempt at CL 153841, which caused two regressions:

1. crypto/ecdsa failed to build with -gcflags=-l=4. This was because
when "t1, t2, ... := g(); f(t1, t2, ...)" was exported, we were losing
the first assignment from the call's Ninit field.

2. net/http/pprof failed to run with -gcflags=-N. This is due to a
conflict with CL 159717: as of that CL, package-scope initialization
statements are executed within the "init.ializer" function, rather
than the "init" function, and the generated temp variables need to be
moved accordingly too.

[Rest of description is as before.]

This CL moves order.go's copyRet logic for rewriting f(g()) into t1,
t2, ... := g(); f(t1, t2, ...) earlier into typecheck. This allows the
rest of the compiler to stop worrying about multi-value functions
appearing outside of OAS2FUNC nodes.

This changes compiler behavior in a few observable ways:

1. Typechecking error messages for builtin functions now use general
case error messages rather than unnecessarily differing ones.

2. Because f(g()) is rewritten before inlining, saved inline bodies
now see the rewritten form too. This could be addressed, but doesn't
seem worthwhile.

3. Most notably, this simplifies escape analysis and fixes a memory
corruption issue in esc.go. See #29197 for details.

Fixes #15992.
Fixes #29197.

Change-Id: I930b10f7e27af68a0944d6c9bfc8707c3fab27a4
Reviewed-on: https://go-review.googlesource.com/c/go/+/166983
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>