When evaluating a constant expression, the gofrontend would incorrectly
    force each operand to be represented as the resulting type before
    checking if the operation was valid with the untyped constants.
    According to the language specification on constant
    expressions(http://golang.org/ref/spec#Constant_expressions): "Untyped
    boolean, numeric, and string constants may be used as operands wherever
    it is legal to use an operand of boolean, numeric, or string type,
    respectively."
    
    Fixes golang/go#11566.
    
    Reviewed-on: https://go-review.googlesource.com/12716

From-SVN: r227758