IR: Metadata/Value split: RAUW in a deterministic order

RAUW in a deterministic order to try to recover the hexagon bot [1],
whose tests started failing once my GCC fixes were in for r223802.

Otherwise, I'm not sure why tests would fail there and not here.

[1]: http://lab.llvm.org:8011/builders/llvm-hexagon-elf/builds/13426