Cranelift: scoped elaboration duplicates pure operations even when there is no partial redundancy to avoid

### `.clif` Test Case

```
test optimize
set opt_level=speed
target x86_64

function %f(i32, i32, i32) -> i32 {
block0(v0: i32, v1: i32, v2: i32):
v3 = imul.i32 v0, v1
brif v2, block1, block2
block1:
return v3
block2:
v4 = iconst.i32 1
v5 = iadd.i32 v3, v4
return v5

}

;; check that the two occurrences of v3 are still the same value after optimization
; check: return v3
; check: iadd.i32 v3, v4
```

### Steps to Reproduce

* `clif-util test meow.clif`

### Expected Results

The `imul` stays in `block0`.

### Actual Results

The `imul` is needlessly duplicated between `block1` and `block2`.

### Versions and Environment

Cranelift version or commit: de29ce35982eba2060709491309ccc93b397168c (main at the time of writing)

Operating system: Linux

Architecture: x86_64