cmd/compile: no need to keep ()'s for all pseudocalls

Minor internal optimization.

Change-Id: I229d4a9df3da542b77f830c2830f63ec23ec1d61
Reviewed-on: https://go-review.googlesource.com/17065Reviewed-by: Chris Manghane <cmang@golang.org>

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

@@ -1386,7 +1386,7 @@ func (p *parser) pseudocall() *Node {
 		defer p.trace("pseudocall")()
 	}
 
-	x := p.pexpr(true) // keep_parens so we can report error below
+	x := p.pexpr(p.tok == '(') // keep_parens so we can report error below
 	switch x.Op {
 	case OCALL:
 		return x