[LoopUnroll][DebugInfo] Don't add metadata to unrolled remainder loop

Debug information can be, and was, corrupted when the runtime
remainder loop was fully unrolled. This is because a !null node can
be created instead of a unique one describing the loop. In this case,
the original node gets incorrectly updated with the NewLoopID
metadata.

In the case when the remainder loop is going to be quickly fully
unrolled, there isn't the need to add loop metadata for it anyway.

Differential Revision: https://reviews.llvm.org/D37338

llvm-svn: 312471