Try to align the point where a large basic block is split.

The split point is picked such that the newly created water has the same
alignment as the function. This makes the island suitable for constant
pool entries with potentially higher alignment.

This also fixes an issue where the basic block was split one instruction
too late, causing nonconvergence of the algorithm.

<rdar://problem/10550705>

There is still an issue with correctly packing differently aligned
entries in the island.

llvm-svn: 146314