stage2: lower 1-byte and 2-byte values saved to stack

* fix handling of `ah`, `bh`, `ch`, and `dh` registers (which are
  actually used as aliases to `dil`, etc. registers). Currenly, we
  treat them as aliases only meaning when we encounter `ah` we make
  sure to set the REX.W to promote the instruction to 64bits and use
  `dil` register instead - otherwise we might have mismatch between
  registers used in different parts of the codegen. In the future,
  we can and should use `ah`, etc. as upper 8bit halves of 16bit
  registers `ax`, etc.
* fix bug in `airCmp` where `.cmp` MIR instruction shouldn't force
  type `Bool` but let the type of the original type propagate downwards
  - we need this to make an informed choice of the target register
  size and hence choose the right encoding down the line.
* implement lowering of 1-byte and 2-byte values to stack and add
  matching stage2 tests for x86_64 codegen

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