We now use __stack_pointer global and global.set/global.get instruction.
This fixes the checking routine for stack_pointer writes accordingly.
This also fixes the existing __stack_pointer test in reg-stackify.ll:
That test used to pass not because of __stack_pointer clashes but
because the function stackpointer_callee was not marked as readnone,
so it was assumed to possibly write to memory arbitraily, and
global.set instruction was marked as mayStore in the .td definition,
so they were identified as intervening writes. After we added readnone
to its attribute, this test fails without this patch.