freedreno/ir3: Improve shader key normalization.

freedreno/ir3: Improve shader key normalization.

We can remove a bunch of conditional code at key comparison time by
computing a bitmask of used key bits at ir3_shader creation time.  This
also gives us a nice place to put additional key simplification to reduce
how many variants we create (like skipping rastflat if we don't read
colors in the FS, or skipping vclamp_color if we don't write colors).

It does mean walking the whole key to AND it, but the key is just 28 bytes
so far so that seems pretty fine.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4562>