Optimize `array.fill` with more libcalls, optimize `array.copy` for more collectors

[alexcrichton]

In #13382 I'm applying an optimization where array.fill for i8-element arrays to be optimized to a memset on the host. This is relatively easy to do because memory.fill already has the infrastructure for this on the host and array.fill is just reusing it. The intended benefit of this is that we get to use the host's vectorized routines for array.fill as opposed to a per-byte-loop within CLIF. This benefit, however, is also theoretically applicable for elements of other sizes (e.g. all the way up to 128-bits). Implementing this, however, would require new libcalls on the host, for example memory.fill{16,32,64,128}.

This is doable without too too much effort, but this was left out of #13382 because it's not clear whether this is worth it. It'd likely be useful to investigate sibling peer compilers to see what they do in the face of array.fill or similar for larger-than-8-bit-types.

[alexcrichton]

I ended up doing a bit more work on #13382 for some more optimizations here. Specifically during array.fill, and array.new_default which uses the same internals, in addition to handling i8 arrays there's a check to see if the initialization value is a constant, and if that constant is a memset-able constant. For example i64.const 0 is a memset-able constant, as well as i64.const -1, but i64.const 1 is not. That enables more usage of memset, notably with array.new_default, which I think is going to be important.

What this does not handle, however, is a few situations:

• We should still vectorize initialization of i64.const 1 in theory, the original premise of this issue
• Due to codegen initializing ref.i31 (i32.const -1), which has the bit-pattern i32.const -1 in CLIF, is not recognized and is not optimized to memset. I think that's due to the fact that the shape at codegen-time is not a constant initialization value but probably instead either the result if an iadd or bor instruction. This is later const-prop'd to a constant, so the final IR looks like it should be memset, but the order of operations didn't go well.
• While unrelated to array.fill, the implementation of array.copy does not use the memory.copy libcall for VMGcRef-based types. This can be used, however, when a GC implementation doesn't have read/write barriers (e.g. the null/copying collectors). In this situation we should ideally make a dynamic deduction based on the collector at compile time and use memory.copy's libcall unconditionally for all types if barriers aren't needed.