Draft: [RFC] nir: Add nir_if_instr for reduce the size of nir_src

What does this MR do and why?

nir: Add nir_if_instr 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 of #9051 (closed) are finished.

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

This can treat as a following work of !22343 (merged)

Signed-off-by: Yonggang Luo luoyonggang@gmail.com