Testing reveals that, on gfx90a, *_load_short_d16_hi does note
preserve the low-order bits of the destination register, even with
SramECC disabled. To prevent the generation of semantically incorrect
code, disable D16PreservesUnesedBits on this target.
(A more specific test may be needed to account for future gfx90*
targets, but checking for gfx90a instructions was what was in the