nir/shrink_vectors: shrink ALU properly

nir/shrink_vectors: shrink ALU properly

ALU instructions of which not all components are read,
can be shrunk to the number of read components.
Previously, this would only remove trailing components.

This patch enables to remove components from any position.

Stat changes for softpipe:
total instructions in shared programs: 3001291 -> 2984698 (-0.55%)
instructions in affected programs: 225585 -> 208992 (-7.36%)
total loops in shared programs: 1389 -> 1358 (-2.23%)
loops in affected programs: 36 -> 5 (-86.11%)

Reviewed-by: Emma Anholt <emma@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/11411>