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>
diff --git a/src/cmd/compile/internal/gc/walk.go b/src/cmd/compile/internal/gc/walk.go
index 04dac7c..f72176b 100644
--- a/src/cmd/compile/internal/gc/walk.go
+++ b/src/cmd/compile/internal/gc/walk.go
@@ -2142,7 +2142,7 @@
 func applywritebarrier(n *Node) *Node {
 	if n.Left != nil && n.Right != nil && needwritebarrier(n.Left, n.Right) {
 		if Debug_wb > 1 {
-			Warnl(int(n.Lineno), "marking %v for barrier", Nconv(n.Left, 0))
+			Warnl(n.Lineno, "marking %v for barrier", Nconv(n.Left, 0))
 		}
 		n.Op = OASWB
 		return n