When doing OBMC prediction, the code must iterate over the blocks
above or to the left of the current block. In reconinter.c and
rdopt.c, there are several pieces of code that do this. These all work
in roughly the same way, iterating over the xd->mi array (although
some are written with for loops and others with do/while). To visit
each neighbouring block exactly once, each of these loops used an
"mi_step" variable which was set to the width or height of the
neighbouring block in mi-units and the loop counter got incremented by
mi_step to jump to the next block.

This patch unifies the code slightly (just using for loops) and
simplifies it when the CHROMA_SUB8X8 experiment is enabled. In this
case, chroma information is stored in the bottom right block of each
8x8 pixel region. That is, if a block has width 4 and an even mi_col,
the chroma information we need is actually found in the block
immediately to its right.

The existing code implemented this by bumping the current column or
row counter (usually mi_col_offset or mi_row_offset) and duplicating
the first part of the loop body to do it again with the new
counter. It also had to double mi_step to avoid visiting the next-door
block again.

The new code essentially just uses the "continue" keyword to restart
the loop. There's a little more book-keeping required: we might have
to increment "ilimit", the maximum loop index, to ensure we don't exit
the loop too early.

The result is hopefully easier to read, but it's also more general (in
the CHROMA_SUB8X8 case). The existing code assumed the current block
never had width or height below 8 and thus mi_col and mi_row were
always even. As such, whenever the neighbouring block had a width or
height of 4, we knew that we needed to skip to the next neighbouring
block to get the required chroma information. This version of the code
can deal with the current block being smaller. The main difference is
that it decides whether to skip forward by examining the parity of
(mi_col + i) or (mi_row + i).

This change will be needed for 16x4/4x16 block support.

Change-Id: I39c1bbc00a6e2ad1ac17b8eed3980a8bcc040074