[X86][SSE] Attempt to pre-truncate arithmetic operations that have already been extended

As discussed on D28219 - it is profitable to combine trunc(binop (s/zext(x), s/zext(y)) to binop(trunc(s/zext(x)), trunc(s/zext(y))) assuming the trunc(ext()) will simplify further

llvm-svn: 292493