fmt: cleanup reflect value handling

fmt: cleanup reflect value handling

Merge printReflectValue into printValue. Determine if handleMethods
was already called in printArg by checking if depth is 0. Do not
call handleMethods on depth 0 again in printValue to not introduce
a performance regression. handleMethods is called already in printArg
to not introduce a performance penalty for top-level Stringer,
GoStringer, Errors and Formatters by using reflect.ValueOf on them
just to retrieve them again as interface{} values in printValue.

Clear p.arg in printValue after handleMethods to print the type
of the value inside the reflect.Value when a bad verb is encountered
on the top level instead of printing "reflect.Value=" as the type of
the argument. This also fixes a bug that incorrectly prints the
whole map instead of just the value for a key if the returned value
by the map for the key is an invalid reflect value.

name                     old time/op  new time/op  delta
SprintfPadding-2          229ns ± 2%   227ns ± 1%  -0.50%  (p=0.013 n=20+20)
SprintfEmpty-2           36.4ns ± 6%  37.2ns ±14%    ~     (p=0.091 n=18+20)
SprintfString-2           102ns ± 1%   102ns ± 0%    ~     (p=0.751 n=20+20)
SprintfTruncateString-2   142ns ± 0%   141ns ± 1%  -0.95%  (p=0.000 n=16+20)
SprintfQuoteString-2      389ns ± 0%   388ns ± 0%  -0.12%  (p=0.019 n=20+20)
SprintfInt-2              100ns ± 2%   100ns ± 1%    ~     (p=0.188 n=20+15)
SprintfIntInt-2           155ns ± 3%   154ns ± 2%    ~     (p=0.092 n=20+20)
SprintfPrefixedInt-2      250ns ± 2%   251ns ± 3%    ~     (p=0.559 n=20+20)
SprintfFloat-2            177ns ± 2%   175ns ± 1%  -1.30%  (p=0.000 n=20+20)
SprintfComplex-2          516ns ± 1%   510ns ± 1%  -1.13%  (p=0.000 n=19+16)
SprintfBoolean-2         90.9ns ± 3%  90.6ns ± 1%    ~     (p=0.193 n=19+19)
SprintfHexString-2        171ns ± 1%   169ns ± 1%  -1.44%  (p=0.000 n=19+20)
SprintfHexBytes-2         180ns ± 1%   180ns ± 1%    ~     (p=0.060 n=19+18)
SprintfBytes-2            330ns ± 1%   329ns ± 1%  -0.42%  (p=0.003 n=20+20)
SprintfStringer-2         354ns ± 3%   352ns ± 3%    ~     (p=0.525 n=20+19)
SprintfStructure-2        804ns ± 3%   776ns ± 2%  -3.56%  (p=0.000 n=20+20)
FprintInt-2               155ns ± 0%   151ns ± 1%  -2.35%  (p=0.000 n=19+20)
FprintfBytes-2            169ns ± 0%   170ns ± 1%  +0.81%  (p=0.000 n=18+19)
FprintIntNoAlloc-2        112ns ± 0%   109ns ± 1%  -2.28%  (p=0.000 n=20+20)

Change-Id: Ib9a39082ed1be0f1f7499ee6fb6c9530f043e43a
Reviewed-on: https://go-review.googlesource.com/20923
Run-TryBot: Rob Pike <r@golang.org>
Reviewed-by: Rob Pike <r@golang.org>