Add Intel encodings for floating point load/store instructions.

Include wasm/*-memory64.cton tests too.

cranelift/filetests/wasm/f64-memory64.cton

@@ -0,0 +1,27 @@
+; Test basic code generation for f64 memory WebAssembly instructions.
+test compile
+
+; We only test on 64-bit since the heap_addr instructions and vmctx parameters
+; explicitly mention the pointer width.
+set is_64bit=1
+isa intel haswell
+
+function %f64_load(i32, i64 vmctx) -> f64 {
+    gv0 = vmctx
+    heap0 = static gv0, min 0x0001_0000, bound 0x0001_0000_0000, guard 0x8000_0000
+
+ebb0(v0: i32, v1: i64):
+    v2 = heap_addr.i64 heap0, v0, 1
+    v3 = load.f64 v2
+    return v3
+}
+
+function %f64_store(f64, i32, i64 vmctx) {
+    gv0 = vmctx
+    heap0 = static gv0, min 0x0001_0000, bound 0x0001_0000_0000, guard 0x8000_0000
+
+ebb0(v0: f64, v1: i32, v2: i64):
+    v3 = heap_addr.i64 heap0, v1, 1
+    store v0, v3
+    return
+}