compiler: Report errors after evaluating invalid constant.

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.

    Reviewed-on: https://go-review.googlesource.com/14646

From-SVN: r229098