[mips] Fix sret arguments for N32/N64 which were accidentally broken in r221534.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221604 91177308-0d34-0410-b5e6-96231b3b80d8
Fix variable names and comment style. NFC.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221608 91177308-0d34-0410-b5e6-96231b3b80d8