[mlir][llvm] Update insertion point handling in LLVM import.

Insert constants and globals in order by maintaining the position
of the constant and global inserted last. Update the tests
to reflect the updated insertion order. Also make sure functions
are always inserted at the end of the module instead of at
the second last position and delete a spurious function in
the intrinsic.ll that seems to exist to avoid the first
function under test ends up at the end of the module.

Reviewed By: ftynse

Differential Revision: https://reviews.llvm.org/D136679