Add x86 SIMD vall_true

In order to implement SIMD's all_true (https://github.com/WebAssembly/simd/blob/master/proposals/simd/SIMD.md#all-lanes-true), we must legalize some instruction (I chose `vall_true`) to a comparison against 0 and a similar reduction as vany_true using `PTEST` and `SETNZ`. Since `icmp` only allows integers but `vall_true` could allow more vector types, `raw_bitcast` is used to convert the lane types into integers, e.g. b32x4 to i32x4. To do so without runtime type-checking, the `raw_bitcast` instruction (which emits no instruction) can now bitcast from any vector type to the same type, e.g. i32x4 to i32x4.

cranelift/codegen/meta/src/shared/instructions.rs

@@ -1639,6 +1639,20 @@ pub(crate) fn define(
         .operands_out(vec![s]),
     );
 
+    ig.push(
+        Inst::new(
+            "vall_true",
+            r#"
+        Reduce a vector to a scalar boolean.
+
+        Return a scalar boolean true if all lanes in ``i`` are non-zero, false otherwise.
+        "#,
+            &formats.unary,
+        )
+        .operands_in(vec![a])
+        .operands_out(vec![s]),
+    );
+
     let x = &operand("x", &TxN.lane_of());
 
     ig.push(