Auto merge of #116270 - cjgillot:gvn-aggregate, r=oli-obk,RalfJung

See through aggregates in GVN

This PR is extracted from rust-lang/rust#111344

The first 2 commit are cleanups to avoid repeated work. I propose to stop removing useless assignments as part of this pass, and let a later `SimplifyLocals` do it. This makes tests easier to read (among others).

The next 3 commits add a constant folding mechanism to the GVN pass, presented in rust-lang/rust#116012. ~This pass is designed to only use global allocations, to avoid any risk of accidental modification of the stored state.~

The following commits implement opportunistic simplifications, in particular:
- projections of aggregates: `MyStruct { x: a }.x` gets replaced by `a`, works with enums too;
- projections of arrays: `[a, b][0]` becomes `a`;
- projections of repeat expressions: `[a; N][x]` becomes `a`;
- transform arrays of equal operands into a repeat rvalue.

Fixes #3090

r? `@oli-obk`

tests/pass/function_pointers.rs

@@ -80,9 +80,8 @@ fn main() {
     // but Miri currently uses a fixed address for monomorphic functions.
     assert!(return_fn_ptr(i) == i);
     assert!(return_fn_ptr(i) as unsafe fn() -> i32 == i as fn() -> i32 as unsafe fn() -> i32);
-    // We don't check anything for `f`. Miri gives it many different addresses
-    // but mir-opts can turn them into the same address.
-    let _val = return_fn_ptr(f) != f;
+    // Miri gives different addresses to different reifications of a generic function.
+    assert!(return_fn_ptr(f) != f);
     // However, if we only turn `f` into a function pointer and use that pointer,
     // it is equal to itself.
     let f2 = f as fn() -> i32;