[TLI] `TargetLowering::SimplifyDemandedVectorElts()`: narrowing bitcast: fill...

[TLI] `TargetLowering::SimplifyDemandedVectorElts()`: narrowing bitcast: fill known zero elts from known src bits

E.g. in
```
%i0 = zext <2 x i8> to <2 x i16>
%i1 = bitcast <2 x i16> to <4 x i8>
```
the `%i0`'s zero bits are known to be `0xFF00` (upper half of every element is known zero),
but no elements are known to be zero, and for `%i1`, we don't know anything about zero bits,
but the elements under `0b1010` mask are known to be zero (i.e. the odd elements).

But, we didn't perform such a propagation.

Noticed while investigating more aggressive `vpmaddwd` formation.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D123163