Fix #82 Elision: match 1 or more returned types

[lverma14]

This is a cleaner way to fix Issue uber-go#82 However, I am unable to account for what reason did
Match in pos.go originally return false in this case. By making this change, there's a risk that
I'm missing out an edge case where
if !got.IsValid() && m.Pos.IsValid() {
should return false but because of the above change would return true & give wrong result.
This passes on all the current testcases and some other tests that I tried but still a risk.

Can test by running

go run ./... -p testdata/patch/return_elision.patch testdata/test_files/test_return_elision.go