[flang][TBAABuilder] not all loads and stores are inside of functions

TBAA builder assumed that all loads/stores are inside of functions and
hit an assertion once it found loads and stores inside of an
omp::ReductionDeclareOp.

For now just don't add TBAA tags to those loads and stores. They would
end up in a different TBAA tree to the host function after
OpenMPIRBuilder inlines them anyway so there isn't an easy way of making
this work.

Commit series for by-ref OpenMP reductions: 2/3

Author: tblah

flang/lib/Optimizer/CodeGen/TBAABuilder.cpp

@@ -102,7 +102,8 @@ void TBAABuilder::attachTBAATag(AliasAnalysisOpInterface op, Type baseFIRType,
     return;
 
   mlir::LLVM::LLVMFuncOp func = op->getParentOfType<mlir::LLVM::LLVMFuncOp>();
-  assert(func && "func.func should have already been converted to llvm.func");
+  if (!func)
+    return;
 
   ++tagAttachmentCounter;
   if (tagAttachmentLimit != kTagAttachmentUnlimited &&