After unrolling a loop with llvm.loop.unroll.count metadata (unroll factor
hint) the loop unroller replaces the llvm.loop.unroll.count metadata with
llvm.loop.unroll.disable metadata to prevent any subsequent unrolling
passes from unrolling more than the hint indicates.  This patch fixes
an issue where loop unrolling could be disabled for other loops as well which
share the same llvm.loop metadata.