Auto merge of #88839 - nbdd0121:alignof, r=nagisa

Introduce NullOp::AlignOf

This PR introduces `Rvalue::NullaryOp(NullOp::AlignOf, ty)`, which will be lowered from `align_of`, similar to `size_of` lowering to `Rvalue::NullaryOp(NullOp::SizeOf, ty)`.

The changes are originally part of #88700 but since it's not dependent on other changes and could have performance impact on its own, it's separated into its own PR.

compiler/rustc_middle/src/mir/mod.rs

@@ -2277,6 +2277,8 @@ impl BinOp {
 pub enum NullOp {
     /// Returns the size of a value of that type
     SizeOf,
+    /// Returns the minimum alignment of a type
+    AlignOf,
     /// Creates a new uninitialized box for a value of that type
     Box,
 }