i965/vec4: Do separate copy followed by constant propagation after opt_vector_float().

i965/vec4: Do separate copy followed by constant propagation after opt_vector_float().

total instructions in shared programs: 5877012 -> 5876617 (-0.01%)
instructions in affected programs:     33140 -> 32745 (-1.19%)

From before the commit that allows VF constant propagation (which hurt
some programs) to here, the results are:

total instructions in shared programs: 5877951 -> 5876617 (-0.02%)
instructions in affected programs:     123444 -> 122110 (-1.08%)

with no programs hurt.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>