CL/aarch64: implement the wasm SIMD v128.load{32,64}_zero instructions.

This patch implements, for aarch64, the following wasm SIMD extensions.

  v128.load32_zero and v128.load64_zero instructions
  https://github.com/WebAssembly/simd/pull/237

The changes are straightforward:

* no new CLIF instructions.  They are translated into an existing CLIF scalar
  load followed by a CLIF `scalar_to_vector`.

* the comment/specification for CLIF `scalar_to_vector` has been changed to
  match the actual intended semantics, per consulation with Andrew Brown.

* translation from `scalar_to_vector` to aarch64 `fmov` instruction.  This
  has been generalised slightly so as to allow both 32- and 64-bit transfers.

* special-case zero in `lower_constant_f128` in order to avoid a
  potentially slow call to `Inst::load_fp_constant128`.

* Once "Allow loads to merge into other operations during instruction
  selection in MachInst backends"
  (https://github.com/bytecodealliance/wasmtime/issues/2340) lands,
  we can use that functionality to pattern match the two-CLIF pair and
  emit a single AArch64 instruction.

* A simple filetest has been added.

There is no comprehensive testcase in this commit, because that is a separate
repo.  The implementation has been tested, nevertheless.

cranelift/codegen/src/isa/aarch64/lower.rs

@@ -837,10 +837,20 @@ pub(crate) fn lower_constant_f128<C: LowerCtx<I = Inst>>(
     rd: Writable<Reg>,
     value: u128,
 ) {
-    let alloc_tmp = |class, ty| ctx.alloc_tmp(class, ty);
-
-    for inst in Inst::load_fp_constant128(rd, value, alloc_tmp) {
-        ctx.emit(inst);
+    if value == 0 {
+        // Fast-track a common case.  The general case, viz, calling `Inst::load_fp_constant128`,
+        // is potentially expensive.
+        ctx.emit(Inst::VecDupImm {
+            rd,
+            imm: ASIMDMovModImm::zero(),
+            invert: false,
+            size: VectorSize::Size8x16,
+        });
+    } else {
+        let alloc_tmp = |class, ty| ctx.alloc_tmp(class, ty);
+        for inst in Inst::load_fp_constant128(rd, value, alloc_tmp) {
+            ctx.emit(inst);
+        }
     }
 }