StructurizeCFG: Add dependency on LowerSwitch pass

Switch instructions were crashing the StructurizeCFG pass, and it's
probably easier anyway if we don't need to handle them in this pass.

Reviewed-by: Christian König <christian.koenig@amd.com>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@191841 91177308-0d34-0410-b5e6-96231b3b80d8