[MLIR][SCF] Create selects from if yield results which are not defined in the body

Previously, the canonicalizer to create ifs from selects would only work
if the if did not have a body other than yielding. This patch upgrade the functionality
to be able to create selects from any if result whose operands are not defined
within the body.

Reviewed By: ftynse

Differential Revision: https://reviews.llvm.org/D121943