[mlir][LLVM] Support opaque pointers in data layout entries

[zero9178]

Summary

This is likely preferable to having it crash if one were to specify an opaque pointer type, and the actual element type is unused either way.

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

for (DataLayoutEntryInterface entry : entries) {
     auto key = entry.getKey().get<Type>().cast<LLVMPointerType>();
     auto values = entry.getValue().dyn_cast<DenseIntElementsAttr>();
     if (!values || (values.size() != 3 && values.size() != 4)) {
       return emitError(loc)
              << "expected layout attribute for " << entry.getKey().get<Type>()
              << " to be a dense integer elements attribute with 3 or 4 "
                 "elements";
     }
-    if (!key.getElementType().isInteger(8)) {
+    if (key.getElementType() && !key.getElementType().isInteger(8)) {
       return emitError(loc) << "unexpected layout attribute for pointer to "
                             << key.getElementType();
     }
     if (extractPointerSpecValue(values, DLEntryPos::Abi) >
         extractPointerSpecValue(values, DLEntryPos::Preferred)) {
       return emitError(loc) << "preferred alignment is expected to be at least "
                                "as large as ABI alignment";
     }

mlir/test/Dialect/LLVMIR/layout.mlir

// CHECK: bitsize = 64
     return
   }
 }
 
 // -----
 
 module attributes { dlti.dl_spec = #dlti.dl_spec<
   #dlti.dl_entry<!llvm.ptr<i8>, dense<[32, 32, 64]> : vector<3xi32>>,
-  #dlti.dl_entry<!llvm.ptr<i8, 5>, dense<[64, 64, 64]> : vector<3xi32>>
+  #dlti.dl_entry<!llvm.ptr<i8, 5>, dense<[64, 64, 64]> : vector<3xi32>>,
+  #dlti.dl_entry<!llvm.ptr<4>, dense<[32, 64, 64]> : vector<3xi32>>
 >} {
   // CHECK: @spec
   func.func @spec() {
     // CHECK: alignment = 4
     // CHECK: bitsize = 32
     // CHECK: preferred = 8
     // CHECK: size = 4
     "test.data_layout_query"() : () -> !llvm.ptr<i8>
func.func @spec() {
     // CHECK: preferred = 8
     // CHECK: size = 8
     "test.data_layout_query"() : () -> !llvm.ptr<i8, 5>
     // CHECK: alignment = 4
   // CHECK: bitsize = 32
     // CHECK: preferred = 8
     // CHECK: size = 4
     "test.data_layout_query"() : () -> !llvm.ptr<3>
+    // CHECK: alignment = 8
+  // CHECK: bitsize = 32
+    // CHECK: preferred = 8
+    // CHECK: size = 4
+  "test.data_layout_query"() : () -> !llvm.ptr<4>
     return
   }
 }
 
 // -----
 
 // expected-error@below {{unexpected layout attribute for pointer to 'i32'}}
 module attributes { dlti.dl_spec = #dlti.dl_spec<