AMDGPU/GlobalISel: Fix combine crash because LI is not set in prelegalizer

Caused by legacy min/max combines (select + cmp) asking for legalizer info
in prelegalizer (D135047 added combine to all_combines).
Combine still does not work for AMDGPU since destination opcode is custom,
not legal. Similar combine works on DAG since it asks for legal or custom.

Differential Revision: https://reviews.llvm.org/D137274