Small refactor in desugar of pattern matching

Authored by simonpj on Jul 27 2018, 3:17 AM.

Description

Small refactor in desugar of pattern matching

In reviewing Phab:D4968 for Trac 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