runtime: remove a branch from funcdata

runtime: remove a branch from funcdata

name                   old time/op  new time/op  delta
StackCopyWithStkobj-8  12.1ms ± 7%  11.6ms ± 8%  -3.88%  (p=0.002 n=19+19)

Change-Id: Idf810017d541eba70bcf9c736267de9efae916d5
Reviewed-on: https://go-review.googlesource.com/c/go/+/354072
Trust: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>