[Conversion] Migrate away from PointerUnion::{is,get} (NFC)

Note that PointerUnion::{is,get} have been soft deprecated in
PointerUnion.h:

  // FIXME: Replace the uses of is(), get() and dyn_cast() with
  //        isa<T>, cast<T> and the llvm::dyn_cast<T>

I'm not touching PointerUnion::dyn_cast for now because it's a bit
complicated; we could blindly migrate it to dyn_cast_if_present, but
we should probably use dyn_cast when the operand is known to be
non-null.

Author: kazutakahirata

mlir/lib/Conversion/MemRefToSPIRV/MemRefToSPIRV.cpp

@@ -911,7 +911,7 @@ LogicalResult ReinterpretCastPattern::matchAndRewrite(
     if (auto val = dyn_cast<Value>(offset))
       return val;
 
-    int64_t attrVal = cast<IntegerAttr>(offset.get<Attribute>()).getInt();
+    int64_t attrVal = cast<IntegerAttr>(cast<Attribute>(offset)).getInt();
     Attribute attr = rewriter.getIntegerAttr(intType, attrVal);
     return rewriter.createOrFold<spirv::ConstantOp>(loc, intType, attr);
   }();

mlir/lib/Conversion/MeshToMPI/MeshToMPI.cpp

@@ -241,12 +241,12 @@ struct ConvertUpdateHaloOp
 
     // convert a OpFoldResult into a Value
     auto toValue = [&rewriter, &loc](OpFoldResult &v) {
-      return v.is<Value>()
-                 ? v.get<Value>()
+      return isa<Value>(v)
+                 ? cast<Value>(v)
                  : rewriter.create<::mlir::arith::ConstantOp>(
                        loc,
                        rewriter.getIndexAttr(
-                           cast<IntegerAttr>(v.get<Attribute>()).getInt()));
+                           cast<IntegerAttr>(cast<Attribute>(v)).getInt()));
     };
 
     auto dest = op.getDestination();
@@ -267,11 +267,11 @@ struct ConvertUpdateHaloOp
         getMixedValues(op.getStaticHaloSizes(), op.getHaloSizes(), rewriter);
     // subviews need Index values
     for (auto &sz : haloSizes) {
-      if (sz.is<Value>()) {
-        sz = rewriter
-                 .create<arith::IndexCastOp>(loc, rewriter.getIndexType(),
-                                             sz.get<Value>())
-                 .getResult();
+      if (auto value = dyn_cast<Value>(sz)) {
+        sz =
+            rewriter
+                .create<arith::IndexCastOp>(loc, rewriter.getIndexType(), value)
+                .getResult();
       }
     }
 

mlir/lib/Conversion/VectorToLLVM/ConvertVectorToLLVM.cpp

@@ -124,7 +124,7 @@ static Value getAsLLVMValue(OpBuilder &builder, Location loc,
     return builder.create<LLVM::ConstantOp>(loc, intAttr).getResult();
   }
 
-  return foldResult.get<Value>();
+  return cast<Value>(foldResult);
 }
 
 namespace {