Bug fixes for obmc/ext-inter/ext-tile experiment

Fix 1: in ext-inter + obmc config, properly identify if the left
predictor used for obmc is a compound one in the case that the
neighbor uses wedgeinterinter pred and we will dump the ALTREF part.
This will fix the seg fault in unit test:
VP10/AltRefForcedKeyTestLarge.Frame1IsKey/0

Fix 2: in ext-tile + obmc experiment, handle the case that the
above block does not fit in the same row tile with the current one,
so as to prevent potential crashes.

Change-Id: I1c177d4f4ad15e10d11d8756e146496437753eea