Prevent rotating the blocks of a loop (and thus getting a backedge to be

fallthrough) in cases where we might fail to rotate an exit to an outer
loop onto the end of the loop chain.

Having *some* rotation, but not performing this rotation, is the primary
fix of thep performance regression with -enable-block-placement for
Olden/em3d (a whopping 30% regression). Still working on reducing the
test case that actually exercises this and the new rotation strategy out
of this code, but I want to check if this regresses other test cases
first as that may indicate it isn't the correct fix.

llvm-svn: 145195