This is the first part of a refactor to make vertex compaction optional.
Additionally, it may yield a very small benefit to allocate the PC
space sligtly sooner.

Fossil DB stats on Sienna Cichlid with NGGC on:

Totals from 58239 (45.27% of 128647) affected shaders:
CodeSize: 160502348 -> 160502340 (-0.00%)
Instrs: 30722664 -> 30722662 (-0.00%)
Latency: 137627419 -> 137782218 (+0.11%); split: -0.00%, +0.11%
InvThroughput: 21698587 -> 21699068 (+0.00%); split: -0.00%, +0.00%
Copies: 3288263 -> 3288261 (-0.00%)

Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/12246>