cmd/compile: change two pushedio.bin tests to use importpkg instead

pushedio.bin and importpkg are both non-nil iff we're parsing an
package's export data, so "pushedio.bin == nil" and "importpkg == nil"
are equivalent tests.

Change-Id: I571ee908fef867117ef72c5da1eb24fe9b3fd12d
Reviewed-on: https://go-review.googlesource.com/19751Reviewed-by: Robert Griesemer <gri@golang.org>

src/cmd/compile/internal/gc/lex.go

@@ -1936,7 +1936,7 @@ check:
 		fallthrough
 
 	case '\n':
-		if pushedio.bin == nil {
+		if importpkg == nil {
 			lexlineno++
 		}
 	}
@@ -1948,7 +1948,7 @@ check:
 func ungetc(c int) {
 	curio.peekc1 = curio.peekc
 	curio.peekc = c
-	if c == '\n' && pushedio.bin == nil {
+	if c == '\n' && importpkg == nil {
 		lexlineno--
 	}
 }