go/types: compiling hangs

The following program takes 1+ hours to compile, aborted after that:

``` go
package a
var g = 1e81391777742999
```

Package should either analyze it in a reasonable amount of time (seconds) or give an error if it cannot chew it. Compiling a single line of code for hours does not make sense.

on commit af817890aa7b628234075a6cb01a3a435fb8317d
