[dev.typeparams] cmd/compile: flatten OINLCALL in walk

Inlining replaces inlined calls with OINLCALL nodes, and then somewhat
clumsily tries to rewrite these in place without messing up
order-of-evaluation rules.

But handling these rules cleanly is much easier to do during order,
and escape analysis is the only major pass between inlining and
order. It's simpler to teach escape analysis how to analyze OINLCALL
nodes than to try to hide them from escape analysis.

Does not pass toolstash -cmp, but seems to just be line number
changes.

Change-Id: I1986cea39793e3e1ed5e887ba29d46364c6c532e
Reviewed-on: https://go-review.googlesource.com/c/go/+/332649


Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Trust: Matthew Dempsky <mdempsky@google.com>