nir/copy_propagate: do not copy-propagate MOV srcs with source modifiers

nir/copy_propagate: do not copy-propagate MOV srcs with source modifiers

If a source operand in a MOV has source modifiers, then we cannot
copy-propagate it from the parent instruction and remove the MOV.

v2: remove the check for source modifiers from is_move() (Jason)

v3: Put the check for source modifiers back into is_move() since
    this function is called from copy_prop_alu_src(). Add source
    modifiers checks to is_vec() instead.

Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>