[dev.regabi] cmd/compile: cleanup some leftover cruft

Just clearing away some scaffolding artifacts from previous
refactorings.

[git-generate]
cd src/cmd/compile/internal/gc
rf '
ex {
  import "cmd/compile/internal/ir"
  import "cmd/compile/internal/types"

  var n *ir.Name; n.Name() -> n
  var f *ir.Func; f.Func() -> f

  var o types.Object
  ir.AsNode(o).Sym() -> o.Sym()
  ir.AsNode(o).Type() -> o.Type()
  ir.AsNode(o).(*ir.Name) -> o.(*ir.Name)
  ir.AsNode(o).(*ir.Func) -> o.(*ir.Func)

  var x ir.Node
  ir.AsNode(o) != x -> o != x
}
'

Change-Id: I946ec344bd7ee274900a392da53b95308ceaade4
Reviewed-on: https://go-review.googlesource.com/c/go/+/274592
Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

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

@@ -984,7 +984,7 @@ func clearImports() {
 }
 
 func IsAlias(sym *types.Sym) bool {
-	return sym.Def != nil && ir.AsNode(sym.Def).Sym() != sym
+	return sym.Def != nil && sym.Def.Sym() != sym
 }
 
 // recordFlags records the specified command-line flags to be placed