[mlir][SCF] Loop lb/ub are symbols during Affine Min/Max canonicalization

This fixes a bug in SCF/AffineCanonicalizationUtils.cpp. Loop lb/ub were previously considered dimensions, which caused a crash when a (non-optimizable) affine.min / affine.max expression was processed (due to multiplication of two dims). Lb/ub are now considered symbols and symbols may be multiplied. (The scope of the analysis is "within the loop body", at which point lb/ub are constants.)

Differential Revision: https://reviews.llvm.org/D132021