Revert "cmd/compile: sparse conditional constant propagation"

Revert "cmd/compile: sparse conditional constant propagation"

This reverts CL 483875.

Reason for revert: appears to cause internal compiler errors on the ssacheck builder.

Change-Id: I662418384291470c1962c417797a5890dd9aa7a4
Reviewed-on: https://go-review.googlesource.com/c/go/+/497855
Reviewed-by: Keith Randall <khr@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Bryan Mills <bcmills@google.com>