cmd/compile: flow interface data to heap if CONVIFACE of a non-direct interface escapes

Consider the following code:

func f(x []*T) interface{} {
	return x
}

It returns an interface that holds a heap copy of x (by calling
convT2I or friend), therefore x escape to heap. The current
escape analysis only recognizes that x flows to the result. This
is not sufficient, since if the result does not escape, x's
content may be stack allocated and this will result a
heap-to-stack pointer, which is bad.

Fix this by realizing that if a CONVIFACE escapes and we're
converting from a non-direct interface type, the data needs to
escape to heap.

Running "toolstash -cmp" on std & cmd, the generated machine code
are identical for all packages. However, the export data (escape
tags) differ in the following packages. It looks to me that all
are similar to the "f" above, where the parameter should escape
to heap.

io/ioutil/ioutil.go:118
	old: leaking param: r to result ~r1 level=0
	new: leaking param: r

image/image.go:943
	old: leaking param: p to result ~r0 level=1
	new: leaking param content: p

net/url/url.go:200
	old: leaking param: s to result ~r2 level=0
	new: leaking param: s

(as a consequence)
net/url/url.go:183
	old: leaking param: s to result ~r1 level=0
	new: leaking param: s

net/url/url.go:194
	old: leaking param: s to result ~r1 level=0
	new: leaking param: s

net/url/url.go:699
	old: leaking param: u to result ~r0 level=1
	new: leaking param: u

net/url/url.go:775
	old: (*URL).String u does not escape
	new: leaking param content: u

net/url/url.go:1038
	old: leaking param: u to result ~r0 level=1
	new: leaking param: u

net/url/url.go:1099
	old: (*URL).MarshalBinary u does not escape
	new: leaking param content: u

flag/flag.go:235
	old: leaking param: s to result ~r0 level=1
	new: leaking param content: s

go/scanner/errors.go:105
	old: leaking param: p to result ~r0 level=0
	new: leaking param: p

database/sql/sql.go:204
	old: leaking param: ns to result ~r0 level=0
	new: leaking param: ns

go/constant/value.go:303
	old: leaking param: re to result ~r2 level=0, leaking param: im to result ~r2 level=0
	new: leaking param: re, leaking param: im

go/constant/value.go:846
	old: leaking param: x to result ~r1 level=0
	new: leaking param: x

encoding/xml/xml.go:518
	old: leaking param: d to result ~r1 level=2
	new: leaking param content: d

encoding/xml/xml.go:122
	old: leaking param: leaking param: t to result ~r1 level=0
	new: leaking param: t

crypto/x509/verify.go:506
	old: leaking param: c to result ~r8 level=0
	new: leaking param: c

crypto/x509/verify.go:563
	old: leaking param: c to result ~r3 level=0, leaking param content: c
	new: leaking param: c

crypto/x509/verify.go:615
	old: (nothing)
	new: leaking closure reference c

crypto/x509/verify.go:996
	old: leaking param: c to result ~r1 level=0, leaking param content: c
	new: leaking param: c

net/http/filetransport.go:30
	old: leaking param: fs to result ~r1 level=0
	new: leaking param: fs

net/http/h2_bundle.go:2684
	old: leaking param: mh to result ~r0 level=2
	new: leaking param content: mh

net/http/h2_bundle.go:7352
	old: http2checkConnHeaders req does not escape
	new: leaking param content: req

net/http/pprof/pprof.go:221
	old: leaking param: name to result ~r1 level=0
	new: leaking param: name

cmd/internal/bio/must.go:21
	old: leaking param: w to result ~r1 level=0
	new: leaking param: w

Fixes #29353.

Change-Id: I7e7798ae773728028b0dcae5bccb3ada51189c68
Reviewed-on: https://go-review.googlesource.com/c/162829


Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: David Chase <drchase@google.com>