cmd/compile: change esc.go to use nodeSeq

Move a few local fields all the way to []*Node while I'm at it.

Update #14473.

Change-Id: Ib18360879839ac592f778cf1042f111bdf14add3
Reviewed-on: https://go-review.googlesource.com/20197
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>