i965/fs: Split the GLSL IR -> FS LIR visitor to brw_fs_visitor.cpp.

i965/fs: Split the GLSL IR -> FS LIR visitor to brw_fs_visitor.cpp.

We now have:
brw_fs.cpp handles calling out to everything and optimization.
brw_fs_visitor.cpp handles translating to our LIR.
brw_fs_emit.cpp handles emitting from our LIR to native code.

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