cmd/compile/internal/ir: remove AsNode

Except for a single call site in escape analysis, every use of
ir.AsNode involves a types.Object that's known to contain
an *ir.Name. Asserting directly to that type makes the code simpler
and more efficient.

The one use in escape analysis is extended to handle nil correctly
without it.

Change-Id: I694ae516903e541341d82c2f65a9155e4b0a9809
Reviewed-on: https://go-review.googlesource.com/c/go/+/520775
TryBot-Bypass: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Auto-Submit: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

src/cmd/compile/internal/ir/expr.go

@@ -1110,7 +1110,7 @@ func IsReflectHeaderDataField(l Node) bool {
 func ParamNames(ft *types.Type) []Node {
 	args := make([]Node, ft.NumParams())
 	for i, f := range ft.Params().FieldSlice() {
-		args[i] = AsNode(f.Nname)
+		args[i] = f.Nname.(*Name)
 	}
 	return args
 }