[CodeGen] Fix forward scan in MachineBasicBlock::computeRegisterLiveness.

The scan was incorrectly skipping the first instruction, so a register
could appear to be dead when it was actually live. This eventually leads
to a machine verifier failure and miscompile in arm-ldst-opt.

Differential Revision: https://reviews.llvm.org/D54491

llvm-svn: 346821