ir3: add newly found shlg.b16 instruction

ir3: add newly found shlg.b16 instruction

Example of blob's output:
  (nop3) shlg.b16 hr8.x, (r)8, (r)hr8.x, 12

It does: (src2 << src1) | src2

src1 and src2 could be GPRs, relative GPRs, relative consts,
or immidiates. However, they could not be plain const registers.

Blob does use it in conjuncture with "samgq" instruction.

Signed-off-by: Danylo Piliaiev <dpiliaiev@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/11760>