[ConstraintElim] Don't decompose values wider than 64 bits (#68803)

Our coefficients are 64-bits, so adding/multiplying them can wrap in
64-bits even if there would be no wrapping the full bit width.

The alternative would be to check for overflows during all adds/muls in
decomposition. I assume that we don't particularly care about handling
wide integers here, so I've opted to bail out.

Fixes https://github.com/llvm/llvm-project/issues/68751.