[AMDGPU][MC][GFX10] null cannot be used with 128 bit and wider dst operands

[dpreobra]

Bugzilla Link	43713
Version	trunk
OS	Windows NT
CC	@atamazov,@arsenm

Extended Description

Examples of failed tests:

s_load_dwordx4 null, s[2:3], s0
s_load_dwordx8 null, s[2:3], s0
s_load_dwordx16 null, s[2:3], s0

[dpreobra]

assigned to @dpreobra