compiler: Report errors after evaluating invalid constant.

The fix for golang/go#11541 made the assertion that getting an invalid
Numeric_constant after evaluating an invalid constant must result in
an error in a binary expression.  However, a Numeric_constant can be
invalid if it is unset, which occurs when either operand cannot be
converted to the right type.  These errors are issued after the
Numeric_constant is lowered.

Fixes golang/go#12615.

Change-Id: I54e41d748a325004fde59f9e4e02d9c40f8f6576
Reviewed-on: https://go-review.googlesource.com/14646
Reviewed-by: Ian Lance Taylor <iant@golang.org>

go/expressions.cc

@@ -4909,14 +4909,7 @@ Binary_expression::do_lower(Gogo* gogo, Named_object*,
 	    Numeric_constant nc;
 	    if (!Binary_expression::eval_constant(op, &left_nc, &right_nc,
 						  location, &nc))
-              {
-                if (nc.is_invalid())
-                  {
-                    go_assert(saw_errors());
-                    return Expression::make_error(location);
-                  }
                 return this;
-              }
 	    return nc.expression(location);
 	  }
       }