[ndk][arm] Use vreg while lowering tMOVCCr_pseudo.

The Thumb1 backend is lowering tMOVCCr_pseudo by splitting
the basic blocks and select different variables with phi
instruction.  However, the physical registers can't be used
across the machine basic blocks. Otherwise, there will be
some compile-time fatal error and some run-time incorrect
behavior.

This CL implements a simple live variable analysis to
analyze the usage of physical registers among the splitted
machine basic blocks, so that we can promote them to virtual
registers.

Change-Id: Id9352324e513224b5fb0746543d966934c4ffbdb