machinst x64: implement floating point comparisons

Note that this fixes an encoding issue in which the packed single and packed double prefixes were flipped.

build.rs

@@ -182,7 +182,9 @@ fn experimental_x64_should_panic(testsuite: &str, testname: &str, strategy: &str
     match (testsuite, testname) {
         ("simd", "simd_address") => return false,
         ("simd", "simd_f32x4_arith") => return false,
+        ("simd", "simd_f32x4_cmp") => return false,
         ("simd", "simd_f64x2_arith") => return false,
+        ("simd", "simd_f64x2_cmp") => return false,
         ("simd", "simd_store") => return false,
         ("simd", _) => return true,
         _ => {}