bespillslots: Change spillslot collection API to take size+align
Taking explicit size+po2align makes things easier compared to the previously used ir_type: ir_type* is a middleend concept not everything in the backend is represented as a type/mode anymore (x86_insn_size_t). Furthermore we only respected size+alignment of that type anyway, additional semantics would be unexpectedly ignored. It also simplifies the code in some places. This should also naturally fix a problem in 188.amp where we wrongly passed the type for mode_Ls instead of the type for mode_D before.