Fix LLVM error after lowering thumb tMOVCCr_pseudo.

tMOVCCr_pseudo is implemented by splitting the machine basic
block, a conditional branch, and a phi-function.

The original code will result in register allocation error,
because the physical registers will be used between two
machine basic blocks.  To solve the issue, we have to
compute the live physical registers, create the virtual
registers for them, and replace the usage with the virtual
registers.