compiler: don't try to get backend representation of redefinition

compiler: don't try to get backend representation of redefinition

    Trying to get the backend representation of a redefined name can cause a
    compiler crash as the compiler can walk over the same statements a
    second time.  It's also quite unlikely to produce any additional useful
    error messages for the user.

    Test case follows.  I'm not going to bother adding this test case to the
    testsuite--crash-on-invalid cases are worth fixing but not worth
    continually retesting.

    package p

    type A []int

    func (a A) Sum() (sum int) {
    	for _, v := range a {
    		sum += v
    	}
    	return sum
    }

    type A []int

    func (a A) Sum() (sum int) {
    	for _, v := range a {
    		sum += v
    	}
    	return sum
    }

    Reviewed-on: https://go-review.googlesource.com/30976

From-SVN: r241127