[MLIR] Add getBody() method to SingleImplicitBlockTerminator op trait.

Many ops with this trait have `getBody()` and `getBodyBuilder()` methods defined in `extraClassDeclaration` in tablegen. `getBody()` implementation is the same accross all these ops, but `getBodyBuilder()` can return builders with varying insertion points set. In this PR, `getBody()` is moved into `SingleImplicitBlockTerminator` struct and `getBodyBuilder()` is replaced with `OpBuilder::atBlock(End|Terminator)(op.getBody);`.

Differential Revision: https://reviews.llvm.org/D78864