Eliminate instances of `EmitScalarExpr(E->getArg(n))` in EmitX86BuiltinExpr().

EmitX86BuiltinExpr() emits all args into Ops at the beginning, so don't do that
work again.

This changes behavior: If e.g. ++a was passed as an arg, we incremented a twice
previously. This change fixes that bug.

https://reviews.llvm.org/D50979

llvm-svn: 340348