cmd/compile: fix OADDSTR buffer size calculation
cmd/compile: fix OADDSTR buffer size calculation

The size calculation has been wrong since this code was first committed
in https://golang.org/cl/3120.  The effect was that the compiler always
allocated a temporary buffer on the stack for a non-escaping string
concatenation.  This turns out to make no practical difference, as the
compiler always allocates a buffer of the same size (32 bytes) and the
runtime only uses the temporary buffer if the concatenated strings
fit (check is in rawstringtmp in runtime/string.go).

The effect of this change is to avoid generating a temporary buffer on
the stack that will not be used.

Change-Id: Id632bfe3d6c113c9934c018a2dd4bcbf1784a63d
Reviewed-on: https://go-review.googlesource.com/20112
Run-TryBot: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

mwhudson/go,suncycheng/go,momchil-velikov/go,Cofyc/go,linux-on-ibm-z/go,vanloswang/go,linux-on-ibm-z/go,akutz/go,4ad/go,CAFxX/go,ganboing/go-esx,mwhudson/go,vsekhar/elastic-go,ImJasonH/go,akutz/go,cloudflare/go,momchil-velikov/go,suncycheng/go,momchil-velikov/go,theif519/go,golang/go,linux-on-ibm-z/go,codestation/go,golang/go,codestation/go,CAFxX/go,miolini/go,codestation/go,goodlang/good,ImJasonH/go,polyverse-security/go,josharian/go,nawawi/go,codestation/go,linux-on-ibm-z/go,Cofyc/go,ImJasonH/go,akutz/go,goodlang/good,akutz/go,christopher-henderson/Go,cloudflare/go,linux-on-ibm-z/go,binarycrusader/go,momchil-velikov/go,miolini/go,cloudflare/go,linux-on-ibm-z/go,nawawi/go,miolini/go,suncycheng/go,vsekhar/elastic-go,theif519/go,binarycrusader/go,momchil-velikov/go,derekmarcotte/go,nawawi/go,golang/go,Cofyc/go,polyverse-security/go,CAFxX/go,momchil-velikov/go,codestation/go,binarycrusader/go,linux-on-ibm-z/go,binarycrusader/go,momchil-velikov/go,mwhudson/go,Samurais/go,goodlang/good,golang/go,josharian/go,derekmarcotte/go,vanloswang/go,nawawi/go,theif519/go,polyverse-security/go,vsekhar/elastic-go,Samurais/go,suncycheng/go,vsekhar/elastic-go,theif519/go,polyverse-security/go,golang/go,ganboing/go-esx,goodlang/good,josharian/go,polyverse-security/go,Cofyc/go,golang/go,ganboing/go-esx,golang/go,mwhudson/go,ImJasonH/go,golang/go,akutz/go,josharian/go,Samurais/go,mwhudson/go,binarycrusader/go,4ad/go,ImJasonH/go,4ad/go,vsekhar/elastic-go,CAFxX/go,codestation/go,codestation/go,miolini/go,ganboing/go-esx,miolini/go,vanloswang/go,linux-on-ibm-z/go,derekmarcotte/go,josharian/go,nawawi/go,Samurais/go,vsekhar/elastic-go,4ad/go,vsekhar/elastic-go,akutz/go,CAFxX/go,goodlang/good,akutz/go,CAFxX/go,Cofyc/go,ImJasonH/go,goodlang/good,theif519/go,polyverse-security/go,suncycheng/go,suncycheng/go,cloudflare/go,theif519/go,mwhudson/go,Cofyc/go,ganboing/go-esx,derekmarcotte/go,Cofyc/go,cloudflare/go,Samurais/go,ganboing/go-esx,miolini/go,derekmarcotte/go,derekmarcotte/go,ganboing/go-esx,josharian/go,mwhudson/go,Cofyc/go,miolini/go,4ad/go,4ad/go,nawawi/go,miolini/go,binarycrusader/go,akutz/go,nawawi/go,codestation/go,Samurais/go,derekmarcotte/go,josharian/go,cloudflare/go,cloudflare/go,theif519/go,ganboing/go-esx,vanloswang/go,binarycrusader/go,goodlang/good,suncycheng/go,josharian/go,4ad/go,vanloswang/go,momchil-velikov/go,vanloswang/go,ImJasonH/go,vsekhar/elastic-go,theif519/go,vanloswang/go,ImJasonH/go,Samurais/go,goodlang/good,suncycheng/go,polyverse-security/go,derekmarcotte/go,CAFxX/go,polyverse-security/go,4ad/go,nawawi/go,vanloswang/go,binarycrusader/go,mwhudson/go,cloudflare/go,Samurais/go,CAFxX/go