i965/fs: Move constant propagation to the same codebase as copy prop.

i965/fs: Move constant propagation to the same codebase as copy prop.

This means that we don't get constant prop across into the first block after a
BRW_OPCODE_IF or a BRW_OPCODE_DO, but we have hope for properly doing it
across control flow at some point.  More importantly, with the next commit it
will help avoid O(n^2) with instruction count runtime for shaders that have
many constant moves.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>