[MLIR] Enhance getLargestKnownDivisor for AffineExpr floordiv/ceildiv

The largest known divisor for expressions like (32 * d0 + 32, 128)
ceildiv 8 wasn't being computed tightly; a conservative value of 1 was
being returned. Address this. This leads to a broad improvement for
several affine analyses and rewrites that depend on knowing whether
something is a multiple of a specific constant or such largest known
constant.

Differential Revision: https://reviews.llvm.org/D140185