Draft: nir: Add enum value nir_jump_if for reduce the size of nir_src

Once is_ssa removed from nir_src after

nir: Transition away from nir_register and abs/neg/sat modifiers

And then the is_if will be the extra attributes in nir_src that occupy 8 bytes on x64 system. And there is many nir_src for NIR grach, because indeed nir_src is an edge between SSA nodes. So the amount of nir_src hudge, it's worth to reduce 8 byte for nir_src

And after remove of is_if and parent_if, the handling of nir_src are more consistence. There is many if are removed because of this

After this, we can merge duplicated code about handling if and non-if nir_src at the same time