[Tail duplication] Handle source registers with subregisters

When a block is tail-duplicated, the PHI nodes from that block are
replaced with appropriate COPY instructions. When those PHI nodes
contained use operands with subregisters, the subregisters were
dropped from the COPY instructions, resulting in incorrect code.

Keep track of the subregister information and use this information
when remapping instructions from the duplicated block.

Differential Revision: http://reviews.llvm.org/D19337

llvm-svn: 267583