wasm->CLIF: fn translate_operator: Select/TypedSelect: add missing bitcasts

The translation of Operator::Select and Operator::TypedSelect for vector-typed
operands, lacks the relevant bitcasting of the operands to I8X16.  This commit
adds it.

cranelift/wasmtests/pr2559.wat

@@ -0,0 +1,51 @@
+(module
+  (type (;0;) (func (result v128 v128 v128)))
+
+  (func $main1 (type 0) (result v128 v128 v128)
+    call $main1
+    i8x16.add
+    call $main1
+    i8x16.ge_u
+    i16x8.ne
+    i32.const 13
+    br_if 0 (;@0;)
+    i32.const 43
+    br_if 0 (;@0;)
+    i32.const 13
+    br_if 0 (;@0;)
+    i32.const 87
+    select
+    unreachable
+    i32.const 0
+    br_if 0 (;@0;)
+    i32.const 13
+    br_if 0 (;@0;)
+    i32.const 43
+    br_if 0 (;@0;)
+  )
+  (export "main1" (func $main1))
+
+  (func $main2 (type 0) (result v128 v128 v128)
+    call $main2
+    i8x16.add
+    call $main2
+    i8x16.ge_u
+    i16x8.ne
+    i32.const 13
+    br_if 0 (;@0;)
+    i32.const 43
+    br_if 0 (;@0;)
+    i32.const 13
+    br_if 0 (;@0;)
+    i32.const 87
+    select (result v128)
+    unreachable
+    i32.const 0
+    br_if 0 (;@0;)
+    i32.const 13
+    br_if 0 (;@0;)
+    i32.const 43
+    br_if 0 (;@0;)
+  )
+  (export "main2" (func $main2))
+)