cmd/compile: remove nil check in accessing PAUTOHEAP variable
cmd/compile: remove nil check in accessing PAUTOHEAP variable

CL 23393 introduces PAUTOHEAP, and access of PAUTOHEAP variable is
rewritten to indirection of a PAUTO variable. Mark this variable
non-nil, so this indirection does not introduce extra nil checks.

Change-Id: I31853eed5e60238b6c5bc0546e2e9ab340dcddd9
Reviewed-on: https://go-review.googlesource.com/26831
Run-TryBot: Cherry Zhang <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Keith Randall <8c99c3a9284e493be632950b84cd789d08ed3e9d@golang.org>

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