GlobalISel: split aggregates for PCS lowering

This should match the existing behaviour for passing complicated struct and
array types, in particular HFAs come through like that from Clang.

For C & C++ we still need to somehow support all the weird ABI flags, or at
least those that are present in the IR (signext, byval, ...), and stack-based
parameter passing.

llvm-svn: 281977