panfrost/midgard/disasm: Print 8-bit sources

This handles the usual case. 8-bit register access parallels 16-bit
access, but with one major caveat: in 8-bit mode, only half of the
register file is actually (directly) accessible as sources. In
particular, for each 16-bit integer register (hrN), we can only index a
*single* 8-bit integer (qrN), corresponding to the lower 8-bits. To get
the upper 8-bits, it is required to do an explicit shift. For example,
to add the bytes of a 16-bit integer hr0.x and get the result as an
8-bit qr0, you'd need to do something like:

   ilsr hr1.x, hr0.x, #8
   iadd qr0.x, qr0.x, qr1.x

This scheme diverges from 32-bit registers, in that both the upper and
lower halves of a 32-bit register are individually accessible as a pair
of half registers. For contrast, to add the lower and upper 16-bits of a
32-bit integer r0.x, you can just:

   iadd hr0.x, hr0.x, hr1.x

Since hr1.x = upper 16-bit of r0.x.

Signed-off-by: Alyssa Rosenzweig <[email protected]>

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