CodeGen: use a range-based for loop

Convert a debug assertion into a range-based loop form.  NFC.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@222679 91177308-0d34-0410-b5e6-96231b3b80d8
CodeGen: make i686-windows-itanium more similar to msvc

The itanium environment follows the system calling convention for structures.
Pass small aggregates via registers.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@222680 91177308-0d34-0410-b5e6-96231b3b80d8