[SROA] Unfold gep of index select (#80983)

SROA currently supports converting a gep of select into select of gep if
the select is in the pointer operand. This patch expands support to
selects in an index operand.

This is intended to address the regression reported in
https://github.com/llvm/llvm-project/pull/68882#issuecomment-1924909922.