[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



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@346821 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed