no-copy semantics for if expr

```zig
export fn entry() void {
    var c = true;
    var x = if (c) u8(4) else u32(10);
}
```

```llvm
define void @entry() #2 !dbg !35 {
Entry:
  %c = alloca i1, align 1
  %x = alloca i32, align 4
  store i1 true, i1* %c, align 1, !dbg !44
  call void @llvm.dbg.declare(metadata i1* %c, metadata !39, metadata !DIExpression()), !dbg !45
  %0 = load i1, i1* %c, align 1, !dbg !46
  br i1 %0, label %Then, label %Else, !dbg !46

Then:                                             ; preds = %Entry
  br label %EndIf, !dbg !47

Else:                                             ; preds = %Entry
  br label %EndIf, !dbg !47

EndIf:                                            ; preds = %Else, %Then
  %1 = phi i32 [ 4, %Then ], [ 10, %Else ], !dbg !47
  store i32 %1, i32* %x, align 4, !dbg !47
  call void @llvm.dbg.declare(metadata i32* %x, metadata !42, metadata !DIExpression()), !dbg !48
  ret void, !dbg !49
}
```

src/codegen.cpp

@@ -5606,10 +5606,9 @@ static LLVMValueRef ir_render_instruction(CodeGen *g, IrExecutable *executable,
         case IrInstructionIdUndeclaredIdent:
         case IrInstructionIdCallSrc:
         case IrInstructionIdAllocaSrc:
-            zig_unreachable();
-
+        case IrInstructionIdEndExpr:
         case IrInstructionIdAllocaGen:
-            return nullptr;
+            zig_unreachable();
 
         case IrInstructionIdDeclVarGen:
             return ir_render_decl_var(g, executable, (IrInstructionDeclVarGen *)instruction);