[LCSSA] Don't invalidate SCEV

LCSSA currently invalidates SCEV for all instructions for which
LCSSA phi nodes are created. This used to be necessary, because
SCEV has historically tried to maintain LCSSA form as well. As
such, some SCEV uses of the value would have to become uses of
the phi node instead.

However, nowadays SCEV itself no longer maintains LCSSA form.
The SCEV of on LCSSA phi node will be the same as the SCEV of its
argument. LCSSA is instead maintained in the SCEVExpander.

As such, I believe it is no longer necessary to perform any SCEV
invalidation during LCSSA construction.

After this patch the ScalarEvolution argument to the LCSSA utilities
is no longer necessary -- I'll remove it in a followup NFC patch to
keep this patch more concise.

Differential Revision: https://reviews.llvm.org/D149435