lima/gpir: Use registers for values live in multiple blocks

lima/gpir: Use registers for values live in multiple blocks

This commit adds the framework for cross-basic-block register
allocation. Like ARM's compiler, we assume that the value registers
aren't usable across branches, which means we have to use physical
registers to store any value that crosses a basic block. There are three
parts to this:

1. When translating from NIR, we rely on the NIR out-of-ssa pass to
coalesce values into registers. We insert store_reg instructions for
values used in more than one basic block, and load_reg instructions for
values not defined in the same basic block (or defined after their use,
for loops). So by the time we've translated out of NIR we've already
split things into values (which are only used in the same basic block)
and registers (which are only used in different basic blocks than where
they're defined).

2. We allocate the registers at the same time that we allocate the
values, before the final scheduler. Unlike the values, where the
assigned color is fake, we assign the actual physical index & component
to physregs at this stage. load_reg and store_reg are treated as moves
in the allocator and when creating write-after-read dependencies.

3. Finally, in the main scheduler we have to avoid overwriting existing
live physregs when spilling. First, we have to tell the scheduler which
physical registers are live at the end of each block, to avoid
overwriting those. If a register is only live at the beginning, we can
reuse it for spilling after the last original use in the final program
happens, i.e. before any original use is scheduled, but we have to be
careful to add the proper dependencies so that the spill write is
scheduled before the original reads. To handle this we repurpose
reg_link for uses to be used by the scheduler.

A few register-related things copied over from NIR or from other
drivers can be dropped.

Reviewed-by: Vasily Khoruzhick <anarsoul@gmail.com>