Allow bitwidth difference when checking for isOneOrOneSplat.

This helps handling a case where the BUILD_VECTOR has i16 element type
and i32 constant operands

t2: v8i16 = setcc t8, t17, setult:ch
t3: v8i16 = BUILD_VECTOR Constant:i32<1>, ...
   t4: v8i16 = and t2, t3
      t5: v8i16 = add t8, t4

This can be turned into t5: v8i16 = sub t8, t2, and allows us to remove
t3 and t4 from the DAG.

Differential Revision: https://reviews.llvm.org/D127354