Make `mlir::OpState::operator bool` explicit

[tkoeppe]

Summary

This change makes the conversion of an mlir::OpState to bool explicit. Idiomatic boolean uses continue to work as before, but questionable implicit uses (e.g. accumulating over a range of OpStates to count "true" states) become ill-formed. This makes the class interface a lilttle less error-prone.

I tested this change on our internal (fairly large) codebase, and only one fix was needed, which was ultimately an improvement of the affected code.

[tkoeppe]

I should say that I don't have commit access, so I would welcome any help with committing. Thank you!

mlir/include/mlir/IR/OpDefinition.h

 /// This is the concrete base class that holds the operation pointer and has
 /// non-generic methods that only depend on State (to avoid having them
 /// instantiated on template types that don't affect them.
 ///
 /// This also has the fallback implementations of customization hooks for when
 /// they aren't customized.
 class OpState {
 public:
-  /// Ops are pointer-like, so we allow implicit conversion to bool.
-  operator bool() { return getOperation() != nullptr; }
+  /// Ops are pointer-like, so we allow conversion to bool.
+  explicit operator bool() { return getOperation() != nullptr; }
 
   /// This implicitly converts to Operation*.
   operator Operation *() const { return state; }
 
   /// Shortcut of `->` to access a member of Operation.
   Operation *operator->() const { return state; }
 
   /// Return the operation that this refers to.