Anonymous pointer and slice types are very common and identical
anyway, so just reuse them rather than allocating new ones everywhere
they appear.

Turns out to be a small code/stack size win because SSA relies on
gc.Type identity for reusing temporary stack slots:

   text	   data	    bss	    dec	    hex	filename
6453005	 231643	 146328	6830976	 683b80	go.old
6446660	 231643	 146328	6824631	 6822b7	go.new

Saves on memory usage during compile time too, and maybe a small CPU
time win, but the benchmarks are pretty noisy:

name       old time/op     new time/op     delta
Template       342ms ± 8%      339ms ± 9%    ~           (p=0.332 n=99+99)
Unicode        183ms ± 9%      181ms ±11%    ~           (p=0.274 n=95+98)
GoTypes        1.05s ± 4%      1.04s ± 3%  -1.22%        (p=0.000 n=97+95)
Compiler       4.49s ± 7%      4.46s ± 6%    ~           (p=0.058 n=96+91)

name       old user-ns/op  new user-ns/op  delta
Template        520M ±17%       522M ±20%    ~          (p=0.544 n=98+100)
Unicode         331M ±27%       327M ±30%    ~           (p=0.615 n=98+98)
GoTypes        1.54G ±10%      1.53G ±12%    ~          (p=0.173 n=99+100)
Compiler       6.33G ±10%      6.33G ±10%    ~           (p=0.682 n=98+98)

name       old alloc/op    new alloc/op    delta
Template      44.5MB ± 0%     44.1MB ± 0%  -0.80%        (p=0.000 n=97+99)
Unicode       37.5MB ± 0%     37.3MB ± 0%  -0.44%       (p=0.000 n=98+100)
GoTypes        126MB ± 0%      124MB ± 0%  -1.41%        (p=0.000 n=98+99)
Compiler       518MB ± 0%      508MB ± 0%  -1.90%       (p=0.000 n=98+100)

name       old allocs/op   new allocs/op   delta
Template        441k ± 0%       434k ± 0%  -1.76%       (p=0.000 n=100+97)
Unicode         368k ± 0%       365k ± 0%  -0.69%        (p=0.000 n=99+99)
GoTypes        1.26M ± 0%      1.23M ± 0%  -2.27%       (p=0.000 n=100+99)
Compiler       4.60M ± 0%      4.46M ± 0%  -2.96%       (p=0.000 n=100+99)

Change-Id: I94abce5c57aed0f9c48f567b3ac24c627d4c7c91
Reviewed-on: https://go-review.googlesource.com/30632
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>