[mlir][Vector] Fix a propagation bug with broadcast

In the vector distribute patterns, we used to move
`vector.broadcast`s out of `vector.warp_execute_on_lane0`s
irrespectively of how they were defined.

This could create broadcast operations with invalid semantic.
E.g.,
```
%r = warop ...[32] ... -> vector<1x2xf32> {
  %val = broadcast %in : vector<64xf32> to vetor<1x64xf32>
  vector.yield %val : vector<1x64xf32>
}
```
=>
```
%r = warop ...[32] ... -> vector<64xf32> {
  vector.yield %in : vector<64xf32>
}
// Broadcasting to a narrower type!
broadcast %r : vector<64xf32> to vector<1x2xf32>
```

The root issue is we are trying to broadcast something that is not the same
for each thread, so there is actually nothing to propagate here.

The fix checks that the broadcast we want to create actually makes sense.

Differential Revision: https://reviews.llvm.org/D152154