ac/nir/nggc: Refactor save_reusable_variables.

This makes the code more elegant and also fixes the mistake of
skipping the blocks that come before loops.

Fossil DB changes on Sienna Cichlid with NGGC on:

Totals from 1026 (0.80% of 128647) affected shaders:
SpillSGPRs: 3817 -> 4035 (+5.71%)
CodeSize: 5582856 -> 5538732 (-0.79%); split: -0.89%, +0.10%
Instrs: 1106907 -> 1100180 (-0.61%); split: -0.68%, +0.07%
Latency: 10084948 -> 10052197 (-0.32%); split: -0.37%, +0.05%
InvThroughput: 1567012 -> 1564949 (-0.13%); split: -0.16%, +0.03%
SClause: 39789 -> 39075 (-1.79%); split: -2.33%, +0.54%
Copies: 95184 -> 96456 (+1.34%); split: -0.19%, +1.53%
Branches: 44087 -> 44093 (+0.01%); split: -0.01%, +0.02%
PreSGPRs: 47584 -> 51009 (+7.20%); split: -0.61%, +7.80%

Fixes: 0bb543bb60f93bea5b1c0ed6382ced49e659273e
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/13001>