Change VMMemoryDefinition::current_length to usize (#3134)

* Change VMMemoryDefinition::current_length to `usize`

This commit changes the definition of
`VMMemoryDefinition::current_length` to `usize` from its previous
definition of `u32`. This is a pretty impactful change because it also
changes the cranelift semantics of "dynamic" heaps where the bound
global value specifier must now match the pointer type for the platform
rather than the index type for the heap.

The motivation for this change is that the `current_length` field (or
bound for the heap) is intended to reflect the current size of the heap.
This is bound by `usize` on the host platform rather than `u32` or`
u64`. The previous choice of `u32` couldn't represent a 4GB memory
because we couldn't put a number representing 4GB into the
`current_length` field. By using `usize`, which reflects the host's
memory allocation, this should better reflect the size of the heap and
allows Wasmtime to support a full 4GB heap for a wasm program (instead
of 4GB minus one page).

This commit also updates the legalization of the `heap_addr` clif
instruction to appropriately cast the address to the platform's pointer
type, handling bounds checks along the way. The practical impact for
today's targets is that a `uextend` is happening sooner than it happened
before, but otherwise there is no intended impact of this change. In the
future when 64-bit memories are supported there will likely need to be
fancier logic which handles offsets a bit differently (especially in the
case of a 64-bit memory on a 32-bit host).

The clif `filetest` changes should show the differences in codegen, and
the Wasmtime changes are largely removing casts here and there.

Closes #3022

* Add tests for memory.size at maximum memory size

* Add a dfg helper method

cranelift/wasm/src/code_translator.rs

@@ -2155,6 +2155,9 @@ fn get_heap_addr(
     addr_ty: Type,
     builder: &mut FunctionBuilder,
 ) -> (ir::Value, i32) {
+    // This function will need updates for 64-bit memories
+    debug_assert_eq!(builder.func.dfg.value_type(addr32), I32);
+
     let offset_guard_size: u64 = builder.func.heaps[heap].offset_guard_size.into();
 
     // How exactly the bounds check is performed here and what it's performed