Generalize implicit terminator into an OpTrait

Several groups of operations in different dialects (e.g. AffineForOp,
AffineIfOp; loop::ForOp, loop::IfOp) share the requirement for their regions to
contain 0 or 1 block, and for blocks to always have a specific terminator type.
Furthermore, this terminator may be omitted from the custom syntax.  Generalize
this behavior into OpTrait::SingleBlockImplicitTerminator, parameterized by the
terminator operation type.  This trait provides the verifier that checks the
presence of the terminator, and utility functions adding the terminator in case
of absence.

PiperOrigin-RevId: 258957180