Preserve loop metadata when splitting exit blocks

LoopUtils.cpp contains a utility that splits an loop exit block, so that the new block contains only edges coming from the loop. In the case of nested loops, the exit path for the inner loop might also be the back-edge of the outer loop. The new block which is inserted on this path, is now a latch for the outer loop, and it needs to hold the loop metadata for the outer loop. (The test case gives a more concrete view of the situation.)

Patch by Chang Lin (clin1)

Differential Revision: https://reviews.llvm.org/D53876

llvm-svn: 346810