[mlir][IR] Insert operations before `SingleBlock`'s terminator (#65959)

Change `SingleBlock::{insert,push_back}` to avoid inserting the argument
operation after the block's terminator. This allows removing
`SingleBlockImplicitTerminator`'s functions with the same name.

Define `Block::hasTerminator` checking whether the block has a
terminator operation.

Signed-off-by: Victor Perez <victor.perez@codeplay.com>