i965/vec4: adding vec4_cmod_propagation optimization

i965/vec4: adding vec4_cmod_propagation optimization

vec4 port of fs_cmod_propagation.

Shader-db results (no vec4 grepping):
total instructions in shared programs: 6240413 -> 6235841 (-0.07%)
instructions in affected programs:     401933 -> 397361 (-1.14%)
total loops in shared programs:        1979 -> 1979 (0.00%)
helped:                                2265
HURT:                                  0

v2: remove extra space and combine two if blocks, as suggested by
    Matt Turner
v3: add condition check to bail out if current inst and inst being
    scanned has different writemask, as pointed by Matt Turner
v3: updated shader-db numbers
v4: remove block from foreach_inst_in_block_*_starting_from after
    commit 801f151917fedb13c5c6e96281a18d833dd6901f

Reviewed-by: Matt Turner <mattst88@gmail.com>