LLVM assertion when using immediate values with self by-value

[luqmana]

Using self by-value with immediate values seems to cause an LLVM assertion. Ran into this while working on #6612.

trait Fooable {
    fn yes(self);
}

impl Fooable for int {
    fn yes(self) {
        io::println("Yes");
    }   
}

fn main() {
    21.yes();
}

-> % rustc bar.rs
rustc: /scratch/laden/rust3/src/llvm/lib/IR/Instructions.cpp:2394: static llvm::CastInst* llvm::CastInst::CreatePointerCast(llvm::Value*, llvm::Type*, const llvm::Twine&, llvm::Instruction*): Assertion `S->getType()->isPtrOrPtrVectorTy() && "Invalid cast"' failed.
[1]    601 abort      rustc bar.rs

[jdm]

Same as #5321.