compiler: fix buglet in lshift type determination

compiler: fix buglet in lshift type determination

    It's possible to construct an lshift expression using unsafe.Sizeof
    that is technically a compile-time constant but can't be evaluated
    without going through backend methods. In this case, insure that
    Type::make_non_abstract_type is called on the numeric operand of the
    shift (as opposed to leaving as abstract), to avoid an assert later on
    in the compiler flow.

    Fixes golang/go#21372.

    Reviewed-on: https://go-review.googlesource.com/54370

From-SVN: r251006