Scalar: Don't visit constants in findInnerReductionPhi in LoopInterchange

In LoopInterchange, `findInnerReductionPhi()` looks for reduction
variables, which cannot be constants. Update it to return early in that
case.

This also addresses a blocker for removing use-lists from ConstantData,
whose users could be spread across arbitrary modules in the same
LLVMContext.

Differential Revision: https://reviews.llvm.org/D94712