cmd/internal/obj/x86: estimate text size

cmd/internal/obj/x86: estimate text size

We can’t perfectly predict how large the function
will be, but we can make a safe overestimate.
No significant CPU time changes.

name       old alloc/op    new alloc/op    delta
Template      67.7MB ± 0%     67.5MB ± 0%   -0.24%          (p=0.029 n=4+4)
Unicode       43.9MB ± 0%     43.8MB ± 0%   -0.13%          (p=0.029 n=4+4)
GoTypes        244MB ± 0%      244MB ± 0%   -0.28%          (p=0.029 n=4+4)
Compiler      1.05GB ± 0%     1.05GB ± 0%   -0.38%          (p=0.029 n=4+4)

name       old allocs/op   new allocs/op   delta
Template        795k ± 0%       794k ± 0%   -0.14%          (p=0.029 n=4+4)
Unicode         569k ± 0%       569k ± 0%     ~             (p=0.114 n=4+4)
GoTypes        2.59M ± 0%      2.58M ± 0%   -0.11%          (p=0.029 n=4+4)
Compiler       11.0M ± 0%      11.0M ± 0%   -0.09%          (p=0.029 n=4+4)

Passes toolstash -cmp.

Change-Id: I0a92ab04cba7520540ec58fe7189666d0e771454
Reviewed-on: https://go-review.googlesource.com/20771
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>