Add SIMD bitselect instruction and x86 legalization

This new instructions matches the `bitselect` behavior described in the WASM SIMD spec (https://github.com/WebAssembly/simd/blob/master/proposals/simd/SIMD.md#bitwise-select)

cranelift/codegen/meta/src/isa/x86/legalize.rs

@@ -20,7 +20,9 @@ pub(crate) fn define(shared: &mut SharedDefinitions, x86_instructions: &Instruct
     // List of instructions.
     let insts = &shared.instructions;
     let band = insts.by_name("band");
+    let band_not = insts.by_name("band_not");
     let bitcast = insts.by_name("bitcast");
+    let bitselect = insts.by_name("bitselect");
     let bor = insts.by_name("bor");
     let bnot = insts.by_name("bnot");
     let bxor = insts.by_name("bxor");
@@ -431,6 +433,19 @@ pub(crate) fn define(shared: &mut SharedDefinitions, x86_instructions: &Instruct
         );
     }
 
+    // SIMD select
+    for ty in ValueType::all_lane_types().filter(allowed_simd_type) {
+        let bitselect = bitselect.bind(vector(ty, sse_vector_size)); // must bind both x/y and c
+        narrow.legalize(
+            def!(d = bitselect(c, x, y)),
+            vec![
+                def!(a = band(x, c)),
+                def!(b = band_not(y, c)),
+                def!(d = bor(a, b)),
+            ],
+        );
+    }
+
     narrow.custom_legalize(shuffle, "convert_shuffle");
     narrow.custom_legalize(extractlane, "convert_extractlane");
     narrow.custom_legalize(insertlane, "convert_insertlane");

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

@@ -1199,6 +1199,22 @@ pub(crate) fn define(
         .operands_out(vec![a]),
     );
 
+    let c = &operand_doc("c", Any, "Controlling value to test");
+    ig.push(
+        Inst::new(
+            "bitselect",
+            r#"
+        Conditional select of bits.
+
+        For each bit in `c`, this instruction selects the corresponding bit from `x` if the bit 
+        in `c` is 1 and the corresponding bit from `y` if the bit in `c` is 0. See also: 
+        `select`, `vselect`.
+        "#,
+        )
+        .operands_in(vec![c, x, y])
+        .operands_out(vec![a]),
+    );
+
     let x = &operand("x", Any);
 
     ig.push(