[mips] Remove redundant code in RetCC_MipsN. NFC.

Summary:
i32 is always promoted to i64 so it no longer makes sense to assign i32 to
registers.

Reviewers: vmedic

Reviewed By: vmedic

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D5964

llvm-svn: 220561