* 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.
9f224ebd9a