compiler: Don't crash on invalid arithmetic ops.

The gofrontend would crash after hitting an unreachable state while
trying to determine the type of an arithmetic expression involving
non-numeric values.  Instead of crashing, it should fail gracefully
if the relevant error is already reported.

Fixes golang/go#11537.

Change-Id: I84ec06bb944b8af34ec5e5864a5149c11c782919
Reviewed-on: https://go-review.googlesource.com/13793
Reviewed-by: Ian Lance Taylor <[email protected]>

Author: Chris Manghane

go/expressions.cc

@@ -15150,7 +15150,11 @@ Numeric_constant::set_type(Type* type, bool issue_error, Location loc)
   else if (type->complex_type() != NULL)
     ret = this->check_complex_type(type->complex_type(), issue_error, loc);
   else
-    go_unreachable();
+    {
+      ret = false;
+      if (issue_error)
+        go_assert(saw_errors());
+    }
   if (ret)
     this->type_ = type;
   return ret;