[mips] Check emitted code for llvm.bswap.i32 on MIPS16/MIPS64 and llvm.bswap.i64 on MIPS16.

While reviewing r204163, I noticed that the MIPS16 test only checked for a .ent
directive and didn't actually check the code emitted. Fixed this and added a
check for llvm.bswap.i32 on MIPS64 at the same time.