cmd/compile: eliminate more nil checks of phis

cmd/compile: eliminate more nil checks of phis

The existing implementation started by eliminating
nil checks for OpAddr, OpAddPtr, and OpPhis with
all non-nil args.

However, some OpPhis had all non-nil args,
but their args had not been processed yet.

Pull the OpPhi checks into their own loop,
and repeat until stabilization.

Eliminates a dozen additional nilchecks during make.bash.

Negligible compiler performance impact.

Change-Id: If7b803c3ad7582af7d9867d05ca13e03e109d864
Reviewed-on: https://go-review.googlesource.com/37999
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>