[mlir][arith][NFC] Add missing index_cast fold tests

[kuhar]

Summary

It wasn't immediately obvious to me if index-to-integer casts were
folded or not.

mlir/test/Dialect/Arith/canonicalize.mlir

func.func @indexCastUIOfUnsignedExtend(%arg0: i8) -> index {
   %ext = arith.extui %arg0 : i8 to i16
   %idx = arith.index_castui %ext : i16 to index
   return %idx : index
 }
 
 // CHECK-LABEL: @indexCastFold
 //       CHECK:   %[[res:.*]] = arith.constant -2 : index
 //       CHECK:   return %[[res]]
-func.func @indexCastFold(%arg0: i8) -> index {
+func.func @indexCastFold() -> index {
   %c-2 = arith.constant -2 : i8
   %idx = arith.index_cast %c-2 : i8 to index
   return %idx : index
 }
 
+// CHECK-LABEL: @indexCastFoldIndexToInt
+//       CHECK:   %[[res:.*]] = arith.constant 1 : i32
+//       CHECK:   return %[[res]]
+func.func @indexCastFoldIndexToInt() -> i32 {
+  %c1 = arith.constant 1 : index
+  %int = arith.index_cast %c1 : index to i32
+  return %int : i32
+}
+
 // CHECK-LABEL: @indexCastUIFold
 //       CHECK:   %[[res:.*]] = arith.constant 254 : index
 //       CHECK:   return %[[res]]
-func.func @indexCastUIFold(%arg0: i8) -> index {
+func.func @indexCastUIFold() -> index {
   %c-2 = arith.constant -2 : i8
   %idx = arith.index_castui %c-2 : i8 to index
   return %idx : index
 }
 
+// CHECK-LABEL: @indexCastUIFoldIndexToInt
+//       CHECK:   %[[res:.*]] = arith.constant 1 : i32
+//       CHECK:   return %[[res]]
+func.func @indexCastUIFoldIndexToInt() -> i32 {
+  %c1 = arith.constant 1 : index
+  %int = arith.index_castui %c1 : index to i32
+  return %int : i32
+}
+
 // CHECK-LABEL: @signExtendConstant
 //       CHECK:   %[[cres:.+]] = arith.constant -2 : i16
 //       CHECK:   return %[[cres]]
 func.func @signExtendConstant() -> i16 {
   %c-2 = arith.constant -2 : i8
   %ext = arith.extsi %c-2 : i8 to i16
   return %ext : i16
 }