[DWARF] LLVM ERROR: Broken function found, while removing Debug Intrinsics.

Check that when SimplifyCFG is flattening a 'br', all their debug intrinsic instructions are removed, including any dbg.label referencing a label associated with the basic blocks being removed.

As the test case involves a CFG transformation, move it to the correct location.

Differential Revision: https://reviews.llvm.org/D57444

llvm-svn: 353682

Author: CarlosAlbertoEnciso