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/filetests/src/test_compile.rs

@@ -130,7 +130,10 @@ fn update_test(output: &[&str], context: &Context) -> Result<()> {
             // but after we hit a real line then we push all remaining lines.
             let mut in_next_function = false;
             for line in old_test {
-                if !in_next_function && (line.trim().is_empty() || line.starts_with(";")) {
+                if !in_next_function
+                    && (line.trim().is_empty()
+                        || (line.starts_with(";") && !line.starts_with(";;")))
+                {
                     continue;
                 }
                 in_next_function = true;