While folding unconditional return move DbgRegionEndInst into the predecessor,...

While folding unconditional return move DbgRegionEndInst into the predecessor, instead of removing it. This fixes following tests from llvmgcc42 testsuite.

gcc.c-torture/execute/20000605-3.c
gcc.c-torture/execute/20020619-1.c
gcc.c-torture/execute/20030920-1.c
gcc.c-torture/execute/loop-ivopts-1.c

llvm-svn: 65353