Revive the support of COND_IF's simplified form

Reintroduce support for the custom simplified form of COND_IF to improve
readability.

Additionally, an optional check has been added to TOSA validation pass
in MLIR to ensure that the form of control-flow operations conform to
the specification.

Signed-off-by: TatWai Chong <tatwai.chong@arm.com>
Change-Id: I588e3781e2ebd3b28fa989fa570cf222138146b3