[OpaquePtr] Create new bitcode encoding for atomicrmw

Since the opaque pointer type won't contain the pointee type, we need to
separately encode the value type for an atomicrmw.

Emit this new code for atomicrmw.

Handle this new code and the old one in the bitcode reader.

Reviewed By: dblaikie

Differential Revision: https://reviews.llvm.org/D103123

lib/Bitcode/Writer/BitcodeWriter.cpp

@@ -3103,7 +3103,7 @@ void ModuleBitcodeWriter::writeInstruction(const Instruction &I,
   case Instruction::AtomicRMW:
     Code = bitc::FUNC_CODE_INST_ATOMICRMW;
     pushValueAndType(I.getOperand(0), InstID, Vals); // ptrty + ptr
-    pushValue(I.getOperand(1), InstID, Vals);        // val.
+    pushValueAndType(I.getOperand(1), InstID, Vals); // valty + val
     Vals.push_back(
         getEncodedRMWOperation(cast<AtomicRMWInst>(I).getOperation()));
     Vals.push_back(cast<AtomicRMWInst>(I).isVolatile());