ARM: Fix a bug which was causing convergence failure in constant-island pass.

The bug is in ARMConstantIslands::createNewWater where the upper bound of the
new water split point is computed:

// This could point off the end of the block if we've already got constant
// pool entries following this block; only the last one is in the water list.
// Back past any possible branches (allow for a conditional and a maximally
// long unconditional).
if (BaseInsertOffset + 8 >= UserBBI.postOffset()) {
  BaseInsertOffset = UserBBI.postOffset() - UPad - 8;
  DEBUG(dbgs() << format("Move inside block: %#x\n", BaseInsertOffset));
}

The split point is supposed to be somewhere between the machine instruction that
loads from the constant pool entry and the end of the basic block, before branch
instructions. The code above is fine if the basic block is large enough and
there are a sufficient number of instructions following the machine instruction.
However, if the machine instruction is near the end of the basic block,
BaseInsertOffset can point to the machine instruction or another instruction
that precedes it, and this can lead to convergence failure.

This commit fixes this bug by ensuring BaseInsertOffset is larger than the
offset of the instruction following the constant-loading instruction.

rdar://problem/18581150