See through aggregates in GVN

[cjgillot]

This PR is extracted from #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 #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 rust-lang/miri#3090

r? @oli-obk