[ARM] ARMLoadStoreOpt::UpdateBaseRegUses should stop on def

When UpdateBaseRegUses sees an instruction that defines the base
register it must stop, as the base register value it is updating is no
longer live. Ideally we would already have seen the register be killed
(which is already checked for), but the kill flags may be inaccurate
and we have to account for this.

Differential Revision: http://reviews.llvm.org/D10566

llvm-svn: 240424