cmd/compile: add debug print to identify unexpected objects

This should never be called but should help identify causes of
unexpected panics such as in issue #17716.

Change-Id: Id6ad0cef1088a41bfcc69110a93484a7e39c4128
Reviewed-on: https://go-review.googlesource.com/32480
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>

src/go/types/call.go

@@ -318,6 +318,7 @@ func (check *Checker) selector(x *operand, e *ast.SelectorExpr) {
  x.typ = exp.typ
  x.id = exp.id
  default:
+ check.dump("unexpected object %v (%T)", exp, exp)
  unreachable()
  }
  x.expr = e