nir/vars_to_ssa: Handle missing struct members in foreach_deref_node

nir/vars_to_ssa: Handle missing struct members in foreach_deref_node

This can happen if, for instance, you have an array of structs and there
are both direct and wildcard references to the same struct and some
members only have direct or only have indirect.

Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
Cc: mesa-stable@lists.freedesktop.org
(cherry picked from commit ecf91898e0a8e144adb82d72aecf1224e77ee31b)