In rd_pick_partition(), the first one or two blocks for the partition
types HORZ_A, HORZ_B, VERT_A, and VERT_B may be already evaluated,
during the evaluation of SPLIT, HORZ, and VERT. This patch saves the
RD pick mode results and tries to reuse them to remove the duplicate
RD mode evaluation operations.

This patch should not incur any coding performance loss.

Testing on a few lowres frames: when CFL is off, this patch obtains
>10% encoder speedup.

Change-Id: I932e233bc93873de62a88230254df44494236dde