In reviewing Phab:D4968 for Trac #15385 I saw a small
but simple refactor to avoid unnecessary work in the
desugarer.

This patch just arranges to call
   matchSinglePatVar v ...
rather than
   matchSinglePat (Var v) ...

The more specialised function already existed, as
   match_single_pat_var

I also added more comments about decideBangHood

(cherry picked from commit 45cfe651)