stage2: add zero- and sign-extend moves to x86_64

* remove `LoweringError` error set from `Emit.zig` - it actually
  was less than helpful; it's better to either not throw an error
  since there can be instructions with mismatching operand sizes
  such as `movsx` or assert on a by instruction-basis. Currently,
  let's just pass through and see how we fare.
* when moving integers into registers, check for signedness and move
  with zero- or sign-extension if source operand is smaller than 8
  bytes. The destination operand is always assumed to be full-width,
  i.e., 8 bytes.
* clean up `airTrunc` a little to match the rest of CodeGen inst
  implementations.

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