[SystemZ] Support large LLVM IR struct return values

Recent mesa/llvmpipe crashes on SystemZ due to a failed assertion when
attempting to compile a routine with a return type of
  { <4 x float>, <4 x float>, <4 x float>, <4 x float> }
on a system without vector instruction support.

This is because after legalizing the vector type, we get a return value
consisting of 16 floats, which cannot all be returned in registers.

Usually, what should happen in this case is that the target's CanLowerReturn
routine rejects the return type, in which case SelectionDAG falls back to
implementing a structure return in memory via implicit reference.

However, the SystemZ target never actually implemented any CanLowerReturn
routine, and thus would accept any struct return type.

This patch fixes the crash by implementing CanLowerReturn.  As a side effect,
this also handles fp128 return values, fixing a todo that was noted in
SystemZCallingConv.td.

llvm-svn: 244889