cmd/compile: invalidate CFG when fuseIf triggers

cmd/compile: invalidate CFG when fuseIf triggers

The compiler appears to have a latent bug:
fusePlain calls invalidateCFG when it changes block structure,
but fuseIf does not.

Fix this by hoisting the call to invalidateCFG to the top level.

Change-Id: Ic960fb3ac963b15b4a225aad84863d58efa954e6
Reviewed-on: https://go-review.googlesource.com/c/go/+/177198
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>