cmd/compile: convert fmt.Sprintf to concatenation

cmd/compile: convert fmt.Sprintf to concatenation

There are plenty more, but these cover most of
the trivial cases, and all the cases that
showed up in profiling.

name       old time/op     new time/op     delta
Template       331ms ± 3%      327ms ± 6%    ~           (p=0.143 n=10+10)
Unicode        183ms ± 4%      180ms ± 2%    ~             (p=0.114 n=9+8)
GoTypes        1.12s ± 4%      1.07s ± 1%  -4.34%         (p=0.000 n=10+9)
Compiler       5.16s ± 2%      5.04s ± 2%  -2.24%         (p=0.001 n=10+9)
MakeBash       41.7s ± 2%      42.3s ± 1%  +1.51%        (p=0.000 n=10+10)

name       old alloc/op    new alloc/op    delta
Template      63.4MB ± 0%     63.1MB ± 0%  -0.42%        (p=0.000 n=10+10)
Unicode       43.2MB ± 0%     43.1MB ± 0%  -0.22%         (p=0.000 n=9+10)
GoTypes        220MB ± 0%      219MB ± 0%  -0.57%         (p=0.000 n=8+10)
Compiler       978MB ± 0%      975MB ± 0%  -0.30%        (p=0.000 n=10+10)

name       old allocs/op   new allocs/op   delta
Template        702k ± 0%       686k ± 0%  -2.35%        (p=0.000 n=10+10)
Unicode         548k ± 0%       542k ± 0%  -1.09%        (p=0.000 n=10+10)
GoTypes        2.17M ± 0%      2.09M ± 0%  -3.61%        (p=0.000 n=10+10)
Compiler       9.33M ± 0%      9.15M ± 0%  -1.93%        (p=0.000 n=10+10)

Change-Id: I3a3d7f2d56876427b04cfedc7302d7f496d5bb65
Reviewed-on: https://go-review.googlesource.com/20904
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Dave Cheney <dave@cheney.net>
TryBot-Result: Gobot Gobot <gobot@golang.org>