[MLIR][SCF] Place hoisted scf.if->select prior to the remaining if

This patch slightly updates the behavior of scf.if->select to
place any hoisted select statements prior to the remaining scf.if body.

This allows better composition with other canonicalization passes, such as
scf.if nested merging.

Reviewed By: ftynse

Differential Revision: https://reviews.llvm.org/D122027