As of patch https://aomedia-review.googlesource.com/c/20220 ,
it is no longer always the case that tile_rows == (1 << log2_tile_rows)
and tile_cols == (1 << log2_tile_cols). This exposed an inconsistency
in the tile-group decoding:

When reading the first tile group in a frame, we read
(log2_tile_rows + log2_tile_cols) bits each for the tile group
start and length, but when reading later tile groups we read
get_msb(tile_rows * tile_cols) bits. But now there are edge cases
where those values differ! Since the encoder always uses
(log2_tile_rows + log2_tile_cols) bits, this leads to mis-parsing
the bitstream.

Fix this by moving the decode logic to one function, which always
reads (log2_tile_rows + log2_tile_cols) bits. As a bonus, this gives
us one place to check other invariants, eg. that tile groups can't
run off the end of the frame.

Change-Id: I83b24314526b6055300b70b0f1cdce038e6b23dc