[DAG] Fold vector mul(x,0)/mul(x,1) to a clearing mask

If we're multiplying all elements of a vector by '0' or '1' then we can more efficiently perform this as a clearing mask (that is likely to further simplify to a shuffle blend).

This was noticed when reviewing D87502 but seems to help idiv/irem by constant cases even more as '0'/'1' values are often used for 'passthrough' cases.

Differential Revision: https://reviews.llvm.org/D88225