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.