[DirectX] Bug fix for Data Scalarization crash (#118426)
Two bugs here. First calling `Inst->getFunction()` has undefined
behavior if the instruction is not tracked to a function. I suspect the
`replaceAllUsesWith` was leaving the GEPs in a weird ghost parent
situation. I switched up the visitor to be able to `eraseFromParent` as
part of visiting and then everything started working.
The second bug was in `DXILFlattenArrays.cpp`. I was unaware that you
can have multidimensional arrays of `zeroinitializer`, and `undef` so
fixed up the initializer to handle these two cases.
fixes #117273
