Fix bitcode decoder error in "Encode alignment attribute for atomicrmw"
The wrong record field number was being used in bitcode decoding,
which broke a self-hosted LTO build. (Yet, somehow, this _doesn't_
seem to have broken simple bitcode encode/decode roundtrip tests, and
I'm not sure why...)
Fixes commit d06ab79816785fa362e7d96d7a398bea8064cba7