ac/llvm: convert src operands to pointers if necessary

ac/llvm: convert src operands to pointers if necessary

To avoid generating invalid LLVM IR when both operands don't have
the same type. This might happen when performing pointer comparisons
with SPIRV 1.4.

Fixes invalid LLVM IR for:
dEQP-VK.spirv_assembly.instruction.spirv1p4.opptrequal.variable_pointers_ssbo_equal
dEQP-VK.spirv_assembly.instruction.spirv1p4.opptrnotequal.variable_pointers_ssbo_not_equal

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>