aco/ra: small compile time improvements

This small series adds a flag per SSA value to indicate whether it has been renamed, so that we can avoid most hash map lookups. It also changes the various hash maps to use the arena allocator. Overall seems to speed-up register allocation by about 10-15%, depending on the shader.

I also tried to unify the rename hash maps into a single one using {Temp, block_idx} as Key, but that turned out slower.