cmd/compile: use int32 for line numbers consistently

- removed lots of unnecessary int(x) casts
- removed parserline() - was inconsistently used anyway
- minor simplifications in dcl.go

Change-Id: Ibf7de679eea528a31c9692ef1c76a1d9b3239211
Reviewed-on: https://go-review.googlesource.com/20131
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

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

@@ -483,7 +483,7 @@ func caseClauses(sw *Node, kind int) []*caseClause {
 					break
 				}
 				if Eqtype(c1.node.Left.Type, c2.node.Left.Type) {
-					yyerrorl(int(c2.node.Lineno), "duplicate case %v in type switch\n\tprevious case at %v", c2.node.Left.Type, c1.node.Line())
+					yyerrorl(c2.node.Lineno, "duplicate case %v in type switch\n\tprevious case at %v", c2.node.Left.Type, c1.node.Line())
 				}
 			}
 		}