Auto merge of #138759 - scottmcm:operand-builder, r=saethlin

Allow `enum` and `union` literals to also create SSA values

Today, `Some(x)` always goes through an `alloca`, even in trivial cases where the niching means the constructor doesn't even change the value.

For example, <https://rust.godbolt.org/z/6KG6PqoYz>
```rust
pub fn demo(r: &i32) -> Option<&i32> {
    Some(r)
}
```
currently emits the IR
```llvm
define align 4 ptr `@demo(ptr` align 4 %r) unnamed_addr {
start:
  %_0 = alloca [8 x i8], align 8
  store ptr %r, ptr %_0, align 8
  %0 = load ptr, ptr %_0, align 8
  ret ptr %0
}
```
but with this PR it becomes just
```llvm
define align 4 ptr `@demo(ptr` align 4 %r) unnamed_addr {
start:
  ret ptr %r
}
```
(Of course the optimizer can clean that up, but it'd be nice if it didn't have to -- especially in debug where it doesn't run.  This is like rust-lang/rust#123886, but that only handled non-simd `struct`s -- this PR generalizes it to all non-simd ADTs.)

Doing this means handing variants other than `FIRST_VARIANT`, handling the active field for unions, refactoring the discriminant code so the Place and Operand parts can share the calculation, etc.

Other PRs that led up to this one:
- rust-lang/rust#142005
- rust-lang/rust#142103
- rust-lang/rust#142324
- rust-lang/rust#142383

---

try-job: aarch64-gnu

Author: bors