all: simplify multiple for loops

If a for loop has a simple condition and begins with a simple
"if x { break; }"; we can simply add "!x" to the loop's condition.

While at it, simplify a few assignments to use the common pattern
"x := staticDefault; if cond { x = otherValue(); }".

Finally, simplify a couple of var declarations.

Change-Id: I413982c6abd32905adc85a9a666cb3819139c19f
Reviewed-on: https://go-review.googlesource.com/c/go/+/165342
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/encoding/json/stream_test.go

@@ -296,7 +296,7 @@ type decodeThis struct {
 	v interface{}
 }
 
-var tokenStreamCases []tokenStreamCase = []tokenStreamCase{
+var tokenStreamCases = []tokenStreamCase{
 	// streaming token cases
 	{json: `10`, expTokens: []interface{}{float64(10)}},
 	{json: ` [10] `, expTokens: []interface{}{