[GlobalISel] Preserve IR block layout.

It makes the output function layout more predictable;  the layout has
an effect on performance, we don't want it to be at the mercy of the
translator's visitation order and such.
The predictable output is also easier to digest.

getOrCreateBB isn't appropriately named anymore, as it never needs to
create anything.  Rename it and extract the MBB creation logic out of it.

A couple tests were sensitive to the order. Update them.

llvm-svn: 297868