Add a 2 byte safety margin in offset computations.

Add a 2 byte safety margin in offset computations.

ARMConstantIslandPass still has bugs where jump table compression can
cause constant pool entries to go out of range.

Add a safety margin of 2 bytes when placing constant islands, but use
the real max displacement for verification.

<rdar://problem/11156595>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@153789 91177308-0d34-0410-b5e6-96231b3b80d8