i965/vs: Add constant propagation to a few opcodes.

i965/vs: Add constant propagation to a few opcodes.

This differs from the FS in that we track constants in each
destination channel, and we we have to look at all the swizzled source
channels.  Also, the instruction stream walk is done in an O(n) manner
instead of O(n^2).

Across shader-db, this reduces 8.0% of the instructions from 60.0% of
the vertex shaders, leaving us now behind the old backend by 11.1%
overall.