[MIPS GlobalISel] Lower aggregate structure return arguments

Implement aggregate structure split to simpler types in splitToValueTypes.
splitToValueTypes is used for return values.
According to MipsABIInfo from clang/lib/CodeGen/TargetInfo.cpp,
aggregate structure arguments for O32 always get simplified and thus
will remain unsupported by the MIPS GlobalISel for the time being.
For O32, aggregate structures can be encountered only for complex number
returns e.g. 'complex float' or 'complex double' from <complex.h>.

Differential Revision: https://reviews.llvm.org/D67963

llvm-svn: 372957