Fix bitfield accesses which straddle the boundary of the underlying

type.

 - This generates somewhat less optimal code than before but this is
   not hard to rectify once stable (at the cost of slightly more
   complex code).

 - This currently always uses little-endian ordering of the bitfield. 

 - This breaks the CodeGen/bitfield.c test because it was grepping for
   hard-coded assembly instructions. Will fix once a better test case
   is constructed (hard to do without execution).

 - This fixes SingleSource/UnitTests/2006-01-23-InitializedBitField.c
   and Regression/C/PR1386.c from the test suite.

 - <rdar://problem/6085090>, <rdar://problem/6094169>

llvm-svn: 54395