[PPC64LE] Fix PR24546 - Swap optimization and debug values

This patch fixes PR24546, which demonstrates a segfault during the VSX
swap removal pass.  The problem is that debug value instructions were
not excluded from the list of instructions to be analyzed for webs of
related computation.  I've added the test case from the PR as a crash
test in test/CodeGen/PowerPC.

llvm-svn: 245862