Remove unnecessary check for inalloca in IPConstantPropagation

Summary:
This was added in https://reviews.llvm.org/D2449, but I'm not sure it's
necessary since an inalloca value is never a Constant (should be an
AllocaInst).

Reviewers: hans, rnk

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D79350