[IVUsers] Move preheader check into SCEVExpander

Rather than checking for loop nest preheaders upfront in IVUsers,
move this requirement into isSafeToExpand() from SCEVExpander.

Historically, LSR did not check whether SCEVs are safe to expand
and fully relied on IVUsers to validate this. Later, support for
non-expandable SCEVs was added via rigid formulas.

Checking this in isSafeToExpand() makes it more obvious what
exactly this check is guarding against, and avoids the awkward
loop nest scan.

This is a followup to https://reviews.llvm.org/D111493#3055286.

Differential Revision: https://reviews.llvm.org/D111681