[LICM] Handle store of pointer to itself (PR54495)

Rather than iterating over users and comparing operands, iterate
over uses and check operand number. Otherwise, we'll end up
promoting a store twice if it has two equal operands.

This can only happen with opaque pointers, as otherwise both
operands differ by a level of indirection, so a bitcast would have
to be involved.

Fixes https://github.com/llvm/llvm-project/issues/54495.