switch from ebp to ebx.

ebx encodes more efficiently (1 byte less) for most address modes, than ebp.
previously it was used for 411 format, but the reader uses pinsrw now avoiding
gpr register.

BUG=libyuv:488
R=harryjin@google.com

Review URL: https://codereview.chromium.org/1365003003 .