Fix PR19136: [ARM] Fix Folding SP Update into vpush/vpop

Sicne MBB->computeRegisterLivenes() returns Dead for sub regs like s0,
d0 is used in vpop instead of updating sp, which causes s0 dead before
its use.

This patch checks the liveness of each subreg to make sure the reg is
actually dead.

llvm-svn: 204411