Hoist pure instructions just after side-effectful instructions

When a pure instruction is the *only* use of the result of a side-effectful instruction, we should hoist the pure instruction to just after the side-effectful instruction.

For example, given:

```
...

block42:
...
v123 = load.i8 v36
...

block53:
...
v199 = uextend.i32 v123
...
```

We should hoist the `uextend` to just after the `load` and ensure that the two instructions are in the same block:

```
...

block42:
...
v123 = load.i8 v36
v199 = uextend.i32 v123
...
```

This will ensure that when we decompose a Wasm instruction into smaller RISC-y bits, we can put them back together during instruction selection. If they are *not* in the same block, potentially because of the way that e-graph elaboration delays computing pure values until they are demanded, then we do not consider the `load` to be sinkable into the `uextend`, and we won't be able to put them back together anymore.

cc #8785 and #6154