[mlir][llvm] Fix replaceImmediateSubElements on LLVMPointerType

This method was assearting on opaque pointers because the element is
null.

Depends on D136498

Reviewed By: rriddle

Differential Revision: https://reviews.llvm.org/D136499

mlir/lib/Dialect/LLVMIR/IR/LLVMTypes.cpp

@@ -450,7 +450,7 @@ void LLVMPointerType::walkImmediateSubElements(
 
 Type LLVMPointerType::replaceImmediateSubElements(
     ArrayRef<Attribute> replAttrs, ArrayRef<Type> replTypes) const {
-  return get(replTypes.front(), getAddressSpace());
+  return get(getContext(), replTypes.front(), getAddressSpace());
 }
 
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