Make mlir::OpState::operator bool explicit

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.

Reviewed By: rriddle, mehdi_amini

Differential Revision: https://reviews.llvm.org/D101989

Author: joker-eph

mlir/include/mlir/IR/OpDefinition.h

@@ -93,8 +93,8 @@ void ensureRegionTerminator(
 /// 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; }