[MemorySSA] Extend the clobber walker with the option to skip the starting access.

Summary:
The option enables loop transformations to hoist accesses that do not
have clobbers in the loop. If the clobber queries skips the starting
access, the result may be outside the loop instead of the header Phi.

Adding the walker that uses this option in a separate patch.

Reviewers: george.burgess.iv

Subscribers: sanjoy, jlebar, Prazek, llvm-commits

Differential Revision: https://reviews.llvm.org/D55944

llvm-svn: 350551