MMX argument passing fixes:

On Darwin / Linux x86-32, v8i8, v4i16, v2i32 values are passed in MM[0-2].                                                                                                                                      
On Darwin / Linux x86-32, v1i64 values are passed in memory.                                                                                                                                                    
On Darwin x86-64, v8i8, v4i16, v2i32 values are passed in XMM[0-7].                                                                                                                                     
On Darwin x86-64, v1i64 values are passed in 64-bit GPRs.

llvm-svn: 50257