[mlir] Placeholder used in predicate should be the base type

[Chia-hungDuan]

Summary

Added a notification in the placeholder section. While writing things
like preciate of an attribute, we may embed certain placeholder in the C
expression. Note that the type of the placeholder is only guaranteed to
be the base type like mlir::Type, it's better not to use the derived
type which is based on the implementation.

[Chia-hungDuan]

I'm not sure if we should add a test at the tblgen side, any suggestion?

[mehdi_amini]

mlir/include/mlir/IR/OpBase.td

81

[Chia-hungDuan]

Fix typo

mlir/include/mlir/IR/OpBase.td

 //   E.g., `BoolAttr` is an attribute constraint that wraps a
 //   `CPred<"$_self.isa<BoolAttr>()">` (see the following sections for details).
 //   Then for `F32:$attr`,`$_self` will be replaced by `$attr`.
 //   For type constraints, it's a little bit special since we want the
 //   constraints on each type definition reads naturally and we want to attach
 //   type constraints directly to an operand/result, $_self will be replaced
 //   by the operand/result's type. E.g., for `F32` in `F32:$operand`, its
 //   `$_self` will be expanded as `getOperand(...).getType()`.
+//
+// One thing to be noticed, while using these placeholders in the C expression,
+// the type of placeholder is only guaranteed to be the base type. For example,
+// if you have a predicate in the form `CPred<"CheckType($_self)">, the argument
+// type of the function `CheckType` should be `mlir::Type`.
 class CPred<code pred> : Pred {
   code predExpr = "(" # pred # ")";
 }
 
 // Kinds of predicate combiners.  These must closely match the predicates
 // implemented by the C++ backend (tblgen::PredCombinerKind).
 class PredCombinerKind;
 def PredCombinerAnd : PredCombinerKind;