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 d06ab79816

llvm/lib/Bitcode/Reader/BitcodeReader.cpp

@@ -5181,7 +5181,7 @@ Error BitcodeReader::parseFunctionBody(Function *F) {
       MaybeAlign Alignment;
 
       if (NumRecords == (OpNum + 5)) {
-        if (Error Err = parseAlignmentValue(Record[6], Alignment))
+        if (Error Err = parseAlignmentValue(Record[OpNum + 4], Alignment))
           return Err;
       }