[ModuloSchedule] KernelRewriter::rewrite - silence static analyzer dyn_cast<> null dereference warning. NFCI.
[ModuloSchedule] KernelRewriter::rewrite - silence static analyzer dyn_cast<> null dereference warning. NFCI.

Assert that we've found the start of the MI schedule list.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@372723 91177308-0d34-0410-b5e6-96231b3b80d8

GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm