Revert of cmd/compile: ignore contentEscapes for marking nodes as escaping
Reason for revert: broke the build due to cherrypick;
relies on an unsubmitted parent CL.
Original issue's description:
> cmd/compile: ignore contentEscapes for marking nodes as escaping
>
> We can still stack allocate and VarKill nodes which don't
> escape but their content does.
>
> Fixes #16996
>
> Change-Id: If8aa0fcf2c327b4cb880a3d5af8d213289e6f6bf
> Reviewed-on: https://go-review.googlesource.com/28575
> Run-TryBot: Keith Randall <khr@golang.org>
> TryBot-Result: Gobot Gobot <gobot@golang.org>
> Reviewed-by: David Chase <drchase@google.com>
>