Add a hack in the analyzer to recover some path-sensitivity at branch

conditions. Currently the analyzer does not reason well about
promotions/truncations of symbolic values, so at branch conditions when we see:

  if (condition)
  
and condition is something like a 'short' or 'char', essentially ignore the
promotion to 'int' so that we track constraints on the original symbolic value.
We only ignore the casts if the underlying type has the same or fewer bits as
the converted type.

This fixes:

<rdar://problem/6619921>

llvm-svn: 66899