cmd/compile: remove gratuituous type conversions

Follow-up cleanup for https://go-review.googlesource.com/17248:
Use properly typed local variable op now that that variable use
is not overloaded anymore.

Also: Remove unnecessary if stmt from common lexical path.

Change-Id: I984b0b346f3fdccd5aedc937330c0a5f99acf324
Reviewed-on: https://go-review.googlesource.com/17249
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

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

@@ -92,7 +92,7 @@ type parser struct {
 
 func (p *parser) next() {
 	p.tok = yylex(&p.yy)
-	p.op = Op(p.yy.i)
+	p.op = p.yy.op
 	p.val = p.yy.val
 	p.sym_ = p.yy.sym
 }