I believe this bug can also be triggered by radeonsi though, as it also calls
radeon_llvm_emit_prepare_cube_coords(). When target == TGSI_TEXTURE_CUBE and
opcode == TGSI_OPCODE_TXF, si_shader.c will take and use garbage value from
that function.

Unfortunately I don't have any radeonsi hardware to make a testcase to prove my
point. It might also be difficult anyway due to nature of uinitialized variable
bugs (it's likely to end up with a value that doesn't cause a crash).