With 8 and 16-bit types and anything where we have to use non-trivial strides registersto deal with restrictions, we end up with things that look like partial writes even though we don't care about any values in the register except those written by that instruction. This is particularly important when dealing with loops because liveness sees is_partial_write and the fact that an old version from a previous loop iteration may be valid at that point and extends all purely partially written values to the entire loop.
This commit adds a new UNDEF instruction which does nothing (the generator doesn't emit anything) but which does a fake write to the register. This informs liveness that we don't care about any values before that point so it won't consider those registers to be falsely live. We can safely emit UNDEF instructions for all SSA values that come in from NIR and nearly all temporaries generated by various stages of the compiler. In particular, we need to insert UNDEF instructions when we handle region restrictions because the newly allocated registers are almost guaranteed to be partially written.