Cranelift: Make heap_addr return calculated base + index + offset (#5231)

* Cranelift: Make `heap_addr` return calculated `base + index + offset`

Rather than return just the `base + index`.

(Note: I've chosen to use the nomenclature "index" for the dynamic operand and
"offset" for the static immediate.)

This move the addition of the `offset` into `heap_addr`, instead of leaving it
for the subsequent memory operation, so that we can Spectre-guard the full
address, and not allow speculative execution to read the first 4GiB of memory.

Before this commit, we were effectively doing

    load(spectre_guard(base + index) + offset)

Now we are effectively doing

    load(spectre_guard(base + index + offset))

Finally, this also corrects `heap_addr`'s documented semantics to say that it
returns an address that will trap on access if `index + offset + access_size` is
out of bounds for the given heap, rather than saying that the `heap_addr` itself
will trap. This matches the implemented behavior for static memories, and after
https://github.com/bytecodealliance/wasmtime/pull/5190 lands (which is blocked
on this commit) will also match the implemented behavior for dynamic memories.

* Update heap_addr docs

* Factor out `offset + size` to a helper

cranelift/interpreter/src/interpreter.rs

@@ -1011,7 +1011,7 @@ mod tests {
         block0(v0: i64):
             v1 = iconst.i64 0
             v2 = iconst.i64 123
-            v3 = heap_addr.i64 heap0, v1, 8
+            v3 = heap_addr.i64 heap0, v1, 0, 8
             store.i64 v2, v3
             v4 = load.i64 v3
             v5 = icmp eq v2, v4