i965/fs: Copy propagate partial reads.

i965/fs: Copy propagate partial reads.

This commit reworks copy propagation a bit to support propagating the
copying of partial registers.  This comes up every time we have pull
constants because we do a pull constant read immediately followed by a move
to splat the one component of the out to 8 or 16-wide.  This allows us to
eliminate the copy and simply use the one component of the register.

Shader DB results:

total instructions in shared programs: 5044937 -> 5044428 (-0.01%)
instructions in affected programs:     66112 -> 65603 (-0.77%)
GAINED:                                0
LOST:                                  0

Signed-off-by: Jason Ekstrand <jason.ekstrand@intel.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>