[SCEVExpander] Move LCSSA fixup to ::expand.

Move LCSSA fixup from ::expandCodeForImpl to ::expand(). This has
the advantage that we directly preserve LCSSA nodes here instead of
relying on doing so in rememberInstruction. It also ensures that we
 don't add the non-LCSSA-safe value to InsertedExpressions.

Alternative to D132704.

Fixes #57000.

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D134739