Remove always-true comparison, NFC.

Summary:
Looking at r241279, I noticed that UpgradedIntrinsics only gets written
to in the following code:

    if (UpgradeIntrinsicFunction(&F, NewFn))
      UpgradedIntrinsics[&F] = NewFn;

Looking through UpgradeIntrinsicFunction, we always return false OR
NewFn will be set to a different function from our source.

This patch pulls the F != NewFn into UpgradeIntrinsicFunction as an
assert, and removes the check from callers of UpgradeIntrinsicFunction.

Reviewers: rafael, chandlerc

Subscribers: llvm-commits-list

Differential Revision: http://reviews.llvm.org/D10915

llvm-svn: 241369