Reapply r201180 with an additional error path.

Debug info: Emit values in subregisters that do not have a separate
DWARF register number by emitting a super-register + DW_OP_bit_piece.
This is necessary because on x86_64, there are no DWARF register numbers
for i386-style subregisters.
Fixes a bunch of FIXMEs.

rdar://problem/16015314

llvm-svn: 201190