[PowerPC] During branch relaxation, recompute padding offsets before each iteration

We used to compute the padding contributions to the block sizes during branch
relaxation only at the start of the transformation. As we perform branch
relaxation, we change the sizes of the blocks, and so the amount of inter-block
padding might change. Accordingly, we need to recompute the (alignment-based)
padding in between every iteration on our way toward the fixed point.

Unfortunately, I don't have a test case (and none was provided in the bug
report), and while this obviously seems needed, algorithmically, I don't have
any way of generating a small and/or non-fragile regression test.

llvm-svn: 280626