cmd/compile: add sizeCalculationDisabled flag

cmd/compile: add sizeCalculationDisabled flag

Use it to ensure that dowidth is not called
from the backend on a type whose size
has not yet been calculated.

This is an alternative to CL 42016.

Change-Id: I8c7b4410ee4c2a68573102f6b9b635f4fdcf392e
Reviewed-on: https://go-review.googlesource.com/42018
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>