cmd/compile: simplify converted SSA form for 'if false'

The goal here is to make it easier for a human to
examine the SSA when a function contains lots of dead code.

No significant compiler metric or generated code differences.

Change-Id: I81915fa4639bc8820cc9a5e45e526687d0d1f57a
Reviewed-on: https://go-review.googlesource.com/c/go/+/221791


Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>