[AMDGPU] Simplify liveins in some MIR tests

We can use the following 16-VGPR tuple directly instead of splitting it
into smaller parts:

$vgpr240_vgpr241_vgpr242_vgpr243_vgpr244_vgpr245_vgpr246_vgpr247_vgpr248_vgpr249_vgpr250_vgpr251_vgpr252_vgpr253_vgpr254_vgpr255