[MIR] Unecessary code for some ZST stores are generated

[nagisa]

Code like the following one

use std::marker::PhantomData;

struct Zst { phantom: PhantomData<Zst> }

#[rustc_mir(graphviz="foo.gv")]
fn mir(){
    let x = Zst { phantom: PhantomData };
}

generates some unnecessary LLVM IR. Namely:

bb0:                                              ; preds = %entry-block
  %0 = getelementptr inbounds %Zst, %Zst* %x, i32 0, i32 0
  store %"2.core::marker::PhantomData<Zst>" undef, %"2.core::marker::PhantomData<Zst>"* %0, align 1
  br label %bb1

We should skip zero-sized fields in this code.