glsl/opt_array_splitting: Fix crash when doing array indexing into other arrays

glsl/opt_array_splitting: Fix crash when doing array indexing into other arrays

When we find indirect indexing into an array, the current implementation
of the array spliiting optimization pass does not look further into the
expression tree. However, if the variable expression involves variable
indexing into other arrays, we can miss that these other arrays also have
variable indexing. If that happens, the pass will crash later on after
hitting an assertion put there to ensure that split arrays are in fact
always indexed via constants:

shader_runner: opt_array_splitting.cpp:296:
void ir_array_splitting_visitor::split_deref(ir_dereference**): Assertion `constant' failed.

This patch fixes the problem by letting the pass step into the variable
index expression to identify these cases properly.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=89607
Reviewed-by: Timothy Arceri <timothy.arceri@collabora.com>