[ARM] Search backwards for CMP when combining into CBZ

The constant island pass currently only looks at the instruction immediately
before a branch for a CMP to fold into a CBZ/CBNZ. This extends it to search
backwards for the instruction that defines CPSR. We need to ensure that the
register is not overridden between the CMP and the branch.

Differential Revision: https://reviews.llvm.org/D59317

llvm-svn: 356336