[MLIR][SCF] Canonicalize while statement whose cmp condition is recomputed in the after region

Given a while loop whose condition is given by a cmp, don't recomputed the comparison (or its inverse) in the after region, instead use a constant since  the original condition must be true if we branched to the after region.

Reviewed By: mehdi_amini

Differential Revision: https://reviews.llvm.org/D117047