cmd/gc: synthesize zeroed value for non-assignment context

CL 157910047 introduced code to turn a node representing
a zeroed composite literal into N, the nil Node* pointer
(which represents any zero, not the Go literal nil).

That's great for assignments like x = T{}, but it doesn't work
when T{} is used in a value context like T{}.v or x == T{}.
Fix those.

Should have no effect on performance; confirmed.
The deltas below are noise (compare ns/op):

benchmark                          old ns/op      new ns/op      delta
BenchmarkBinaryTree17              2902919192     2915228424     +0.42%
BenchmarkFannkuch11                2597417605     2630363685     +1.27%
BenchmarkFmtFprintfEmpty           73.7           74.8           +1.49%
BenchmarkFmtFprintfString          196            199            +1.53%
BenchmarkFmtFprintfInt             213            217            +1.88%
BenchmarkFmtFprintfIntInt          336            356            +5.95%
BenchmarkFmtFprintfPrefixedInt     289            294            +1.73%
BenchmarkFmtFprintfFloat           415            416            +0.24%
BenchmarkFmtManyArgs               1281           1271           -0.78%
BenchmarkGobDecode                 10271734       10307978       +0.35%
BenchmarkGobEncode                 8985021        9079442        +1.05%
BenchmarkGzip                      410233227      412266944      +0.50%
BenchmarkGunzip                    102114554      103272443      +1.13%
BenchmarkHTTPClientServer          45297          44993          -0.67%
BenchmarkJSONEncode                19499741       19498489       -0.01%
BenchmarkJSONDecode                76436733       74247497       -2.86%
BenchmarkMandelbrot200             4273814        4307292        +0.78%
BenchmarkGoParse                   4024594        4028937        +0.11%
BenchmarkRegexpMatchEasy0_32       131            135            +3.05%
BenchmarkRegexpMatchEasy0_1K       328            333            +1.52%
BenchmarkRegexpMatchEasy1_32       115            117            +1.74%
BenchmarkRegexpMatchEasy1_1K       931            948            +1.83%
BenchmarkRegexpMatchMedium_32      216            217            +0.46%
BenchmarkRegexpMatchMedium_1K      72669          72857          +0.26%
BenchmarkRegexpMatchHard_32        3818           3809           -0.24%
BenchmarkRegexpMatchHard_1K        121398         121945         +0.45%
BenchmarkRevcomp                   613996550      615145436      +0.19%
BenchmarkTemplate                  93678525       93267391       -0.44%
BenchmarkTimeParse                 414            411            -0.72%
BenchmarkTimeFormat                396            399            +0.76%


Fixes issue 8947.

LGTM=r
R=r, dave
CC=golang-codereviews
https://codereview.appspot.com/162130043