i965/vec4: Change types as needed to propagate source modifiers using current instruction

i965/vec4: Change types as needed to propagate source modifiers using current instruction

SEL and MOV instructions, as long as they don't have source modifiers, are
just copying bits around.  So those kind of instruction could be propagated
even if there are type mismatches. This is needed because NIR generates
integer SEL and MOV instructions whenever it doesn't know what else to
generate.

This commit adds support for copy propagation using current instruction
as reference.

Equivalent to commit 472ef9 but for vec4.

v2: include check for saturate, as Jason Ekstrand suggested
v3: check that the dst.type and the src type are the same, in order to
    solve (among others) the following deqp regression with v2:
    dEQP-GLES3.functional.shaders.operator.unary_operator.minus.lowp_uint_vertex

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