[SimplifyCFG] Don't create PHI nodes for constant bundle operands

Summary:
Constant bundle operands may need to retain their constant-ness for
correctness.  I'll admit that this is slightly odd, but it looks like
SimplifyCFG already does this for things like @llvm.frameaddress and
@llvm.stackmap, so I suppose adding one more case is not a big deal.

It is possible to add a mechanism to denote bundle operands that need to
remain constants, but that's probably too complicated for the time
being.

Reviewers: jmolloy

Subscribers: mcrosier, llvm-commits

Differential Revision: https://reviews.llvm.org/D25502

llvm-svn: 284028