[MLIR] Simplify predicate in Matchers.h, NFC

The ConstantLike trait already static_asserts that operations
implementing it have a single result and zero operands, so we
don't need to redundantly check in Matchers.h

The static assert is in `class ConstantLike` in OpDefinition.h

Differential Revision: https://reviews.llvm.org/D141783