Plan for new slot computations:

- reverse the allocation of all stack slots
  so that isel slots go below spill locations
- the fast allocs must be allocated by decreasing
  alignment constraints
- instead of the svec vector, we simply need two
  numbers that contain the size of both the locals
  and spills (stk0, stk1)
- maybe it's time to include small alignments
- use Tmp.spill == -1 to mark unallocated slots

Layout:

  ---------- rbp = 0 [16]
  | << padding 1 >>
  | .. spills   ..
  |                <- enforce align 16
  | .. align 16 ..
  | .. align  8 ..
  | .. align  4 ..
  | << padding 0 >>
  ---------- rsp = 0 [16]

  padding 0: inserted at last minute by the code
             emitter to respect the ABI
  padding 1: inserted at the beginning of spill
             it can be 4 or 0

Examples:

  if the first local is aligned 4 of size 4, its slot
  number will be set 0, to emit the proper offset to