Add i64x2 integer multiplication using AVX512DQ

filetests/isa/x86/simd-avx512-arithmetic-binemit.clif

@@ -0,0 +1,17 @@
+test binemit
+set enable_simd
+target x86_64 skylake has_avx512dq=true
+
+function %imul_i64x2() {
+block0:
+    [-, %xmm1]    v0 = vconst.i64x2 [1 2]
+    [-, %xmm2]    v1 = vconst.i64x2 [2 2]
+    [-, %xmm19]   v2 = imul v0, v1 ; bin: 62 e2 f5 08 40 da
+    ; 62, mandatory EVEX prefix
+    ; e2 = 1110 0010, R, X, B are unset (inverted) while R' is set (MSB in %xmm19); mm is set to 0F38
+    ; f5 = 1111 0101, W is set (64-bit op), vvvv set to 1 (inverted), bit 2 always set, pp set to 01
+    ; 08 = 0000 1000, everything, LL' indicates 128-bit, V' is unset (inverted, %xmm1 has MSB of 0)
+    ; 40, opcode (correct)
+    ; da = 1100 1010, ModR/M byte using 0b011 from %xmm19 in reg and 0b010 from %xmm2 in r/m
+    return
+}