clif: Remove the type variable from swizzle (#5897)

This instruction is only defined with i8x16 inputs and outputs so
there's no need for a type variable, so shadow the otherwise-generic `a`
result with a concrete i8x16 type.

cranelift/filetests/filetests/isa/x64/simd-lane-access-compile.clif

@@ -114,7 +114,7 @@ function %swizzle() -> i8x16 {
 block0:
     v0 = vconst.i8x16 [0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15]
     v1 = vconst.i8x16 [0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15]
-    v2 = swizzle.i8x16 v0, v1
+    v2 = swizzle v0, v1
     return v2
 }