[AArch64CompressJumpTables] Prevent over-compression caused by invalid alignment.

AArch64CompressJumpTables assumes it can calculate exact block
offsets. This assumption is bogus because getInstSizeInBytes()
only returns an upper bound rather than an exact size. The
assumption is also invalid when a block alignment is bigger than
the function's alignment.

To mitigate both scenarios this patch changes the algorithm to
compute the maximum upper bound for all block offsets. This is
pessimistic but safe because all offsets are treated as unsigned.

Differential Revision: https://reviews.llvm.org/D150009