Simplify REG_SEQUENCE lowering.

The TwoAddressInstructionPass takes the machine code out of SSA form by
expanding REG_SEQUENCE instructions into copies. It is no longer
necessary to rewrite the registers used by a REG_SEQUENCE instruction
because the new coalescer algorithm can do it now.

REG_SEQUENCE is just converted to a sequence of sub-register copies now.

llvm-svn: 169067