[SLP]Fix a crash when reorder masked gather nodes with reused scalars.

If the masked gather nodes must be reordered, we can just reorder
scalars, just like for gather nodes. But if the node contains reused
scalars, it must be handled same way as a regular vectorizable node,
since need to reorder reused mask, not the scalars directly.

Differential Revision: https://reviews.llvm.org/D128360