[mlir] SCFToStandard: support any ops in and around the control flow ops

Originally, the SCFToStandard conversion only declared Ops from the Standard
dialect as legal after conversion. This is undesirable as it would fail the
conversion if the SCF ops contained ops from any other dialect. Furthermore,
this would be problematic for progressive lowering of `scf.parallel` to
`scf.for` after `ensureRegionTerminator` is made aware of the pattern rewriting
infrastructure because it creates temporary `scf.yield` operations declared
illegal. Change the legalization target to declare any op other than `scf.for`,
`scf.if` and `scf.parallel` legal.

Differential Revision: https://reviews.llvm.org/D80137