We can produce such vectors in the Promote Alloca pass,
but we are unable to use movrel to operate it and lower
via scratch. Making it legal makes SI_INDIRECT patterns
work.
There is more work to do in subsequent changes:
- We initialize m0 twice to access each dword. It shall
be possible to only do it once and increment base register
number instead.
- We also need v16i64/v16f64 but these first need to be
added to tablegen.