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 <cdc248cc4d32e4fbfe04e45228163bcecd48a0b9@gmail.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: David Chase <ae171625adc8645b8b79b7e405a6da5aba5341e0@google.com>

Cofyc/go,derekmarcotte/go,josharian/go,vsekhar/elastic-go,vsekhar/elastic-go,CAFxX/go,derekmarcotte/go,CAFxX/go,linux-on-ibm-z/go,codestation/go,golang/go,nawawi/go,ImJasonH/go,cloudflare/go,codestation/go,derekmarcotte/go,polyverse-security/go,cloudflare/go,golang/go,josharian/go,christopher-henderson/Go,vsekhar/elastic-go,polyverse-security/go,linux-on-ibm-z/go,polyverse-security/go,linux-on-ibm-z/go,akutz/go,Cofyc/go,golang/go,Cofyc/go,nawawi/go,ImJasonH/go,cloudflare/go,polyverse-security/go,nawawi/go,derekmarcotte/go,polyverse-security/go,Cofyc/go,josharian/go,derekmarcotte/go,akutz/go,golang/go,golang/go,polyverse-security/go,derekmarcotte/go,codestation/go,akutz/go,CAFxX/go,josharian/go,linux-on-ibm-z/go,vsekhar/elastic-go,codestation/go,ImJasonH/go,codestation/go,akutz/go,linux-on-ibm-z/go,josharian/go,cloudflare/go,akutz/go,derekmarcotte/go,akutz/go,polyverse-security/go,nawawi/go,CAFxX/go,vsekhar/elastic-go,linux-on-ibm-z/go,vsekhar/elastic-go,nawawi/go,Cofyc/go,ImJasonH/go,josharian/go,akutz/go,Cofyc/go,linux-on-ibm-z/go,josharian/go,ImJasonH/go,codestation/go,linux-on-ibm-z/go,golang/go,codestation/go,polyverse-security/go,ImJasonH/go,codestation/go,nawawi/go,Cofyc/go,vsekhar/elastic-go,CAFxX/go,cloudflare/go,cloudflare/go,golang/go,cloudflare/go,cloudflare/go,akutz/go,Cofyc/go,CAFxX/go,derekmarcotte/go,ImJasonH/go,nawawi/go,ImJasonH/go,josharian/go,vsekhar/elastic-go,golang/go,CAFxX/go,CAFxX/go,nawawi/go