cmd/compile: make only one new Node in defaultlit

cmd/compile: make only one new Node in defaultlit

defaultlit and friends sometimes create a new
OLITERAL node, only to have replace it.
Thread hints when that is unnecessary.

name       old time/op     new time/op     delta
Template       318ms ± 6%      322ms ± 4%     ~           (p=0.154 n=24+25)
Unicode        162ms ± 6%      151ms ± 7%   -6.94%        (p=0.000 n=22+23)
GoTypes        1.04s ± 1%      1.04s ± 3%     ~           (p=0.136 n=20+25)
Compiler       5.08s ± 2%      5.10s ± 4%     ~           (p=0.788 n=25+25)
MakeBash       41.4s ± 1%      41.5s ± 1%     ~           (p=0.084 n=25+25)

name       old user-ns/op  new user-ns/op  delta
Template        438M ±10%       441M ± 9%     ~           (p=0.418 n=25+25)
Unicode         272M ± 5%       219M ± 5%  -19.33%        (p=0.000 n=24+21)
GoTypes        1.51G ± 3%      1.51G ± 3%     ~           (p=0.500 n=25+25)
Compiler       7.31G ± 3%      7.32G ± 3%     ~           (p=0.572 n=25+24)

name       old alloc/op    new alloc/op    delta
Template      57.3MB ± 0%     57.2MB ± 0%   -0.16%        (p=0.000 n=25+25)
Unicode       41.1MB ± 0%     38.7MB ± 0%   -5.81%        (p=0.000 n=25+25)
GoTypes        191MB ± 0%      191MB ± 0%   -0.06%        (p=0.000 n=25+25)
Compiler       840MB ± 0%      839MB ± 0%   -0.12%        (p=0.000 n=25+25)

name       old allocs/op   new allocs/op   delta
Template        500k ± 0%       500k ± 0%   -0.12%        (p=0.000 n=24+25)
Unicode         400k ± 0%       384k ± 0%   -4.16%        (p=0.000 n=25+25)
GoTypes        1.50M ± 0%      1.49M ± 0%   -0.05%        (p=0.000 n=25+25)
Compiler       6.04M ± 0%      6.03M ± 0%   -0.11%        (p=0.000 n=25+25)

Change-Id: I2fda5e072db67ba239848bde827c7deb2ad4abae
Reviewed-on: https://go-review.googlesource.com/20813
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>