[LoopStrengthReduction] Fix pointer extend asserts

Additional asserts were added to ScalarEvolution to enforce
pointer/int type rules. An assert is triggered when the LSR pass
attempts to extend a pointer SCEV in GenerateTruncates.

This patch changes GenerateTruncates to exit early if the Formaula
contains a ScaledReg or BaseReg with a pointer type.

Differential Revision: https://reviews.llvm.org/D107185