[MLIR] Simplify affine.if having yield values and trivial conditions

When an affine.if operation is returning/yielding results and has a
trivially true or false condition, then its 'then' or 'else' block,
respectively, is promoted to the affine.if's parent block and then, the
affine.if operation is replaced by the correct results/yield values.
Relevant test cases are also added.

Signed-off-by: Srishti Srivastava <srishti.srivastava@polymagelabs.com>

Differential Revision: https://reviews.llvm.org/D105418