Improved vp9_ihtllm_c

Improved vp9_ihtllm_c

As suggested by Yaowu, we can use eob to reduce the complexity
of the vp9_ihtllm_c function.  For the 1080p test clip used, the decoder
performance improved by 17%.

Change-Id: I32486f2f06f9b8f60467d2a574209aa3a3daa435