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 <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>