Consolidate address calculations for atomics (#3143)

* Consolidate address calculations for atomics

This commit consolidates all calcuations of guest addresses into one
`prepare_addr` function. This notably remove the atomics-specifics paths
as well as the `prepare_load` function (now renamed to `prepare_addr`
and folded into `get_heap_addr`).

The goal of this commit is to simplify how addresses are managed in the
code generator for atomics to use all the shared infrastrucutre of other
loads/stores as well. This additionally fixes #3132 via the use of
`heap_addr` in clif for all operations.

I also added a number of tests for loads/stores with varying alignments.
Originally I was going to allow loads/stores to not be aligned since
that's what the current formal specification says, but the overview of
the threads proposal disagrees with the formal specification, so I
figured I'd leave it as-is but adding tests probably doesn't hurt.

Closes #3132

* Fix old backend

* Guarantee misalignment checks happen before out-of-bounds

tests/all/wast.rs

@@ -25,6 +25,11 @@ fn run_wast(wast: &str, strategy: Strategy, pooling: bool) -> anyhow::Result<()>
     // by reference types.
     let reftypes = simd || wast.iter().any(|s| s == "reference-types");
 
+    // Threads aren't implemented in the old backend, so skip those tests.
+    if threads && cfg!(feature = "old-x86-backend") {
+        return Ok(());
+    }
+
     let mut cfg = Config::new();
     cfg.wasm_simd(simd)
         .wasm_bulk_memory(bulk_mem)