x64: lower i64x2.imul to VPMULLQ when possible

This adds the machinery to encode the VPMULLQ instruction which is
available in AVX512VL and AVX512DQ. When these feature sets are
available, we use this instruction instead of a lengthy 12-instruction
sequence.

cranelift/codegen/src/isa/x64/inst/emit_tests.rs

@@ -3555,6 +3555,12 @@ fn test_x64_emit() {
         "pmullw  %xmm14, %xmm1",
     ));
 
+    insns.push((
+        Inst::xmm_rm_r_evex(Avx512Opcode::Vpmullq, RegMem::reg(xmm14), xmm10, w_xmm1),
+        "62D2AD0840CE",
+        "vpmullq %xmm14, %xmm10, %xmm1",
+    ));
+
     insns.push((
         Inst::xmm_rm_r(SseOpcode::Pmuludq, RegMem::reg(xmm8), w_xmm9),
         "66450FF4C8",
@@ -4283,6 +4289,7 @@ fn test_x64_emit() {
     isa_flag_builder.enable("has_ssse3").unwrap();
     isa_flag_builder.enable("has_sse41").unwrap();
     isa_flag_builder.enable("has_avx512f").unwrap();
+    isa_flag_builder.enable("has_avx512dq").unwrap();
     let isa_flags = x64::settings::Flags::new(&flags, isa_flag_builder);
 
     let rru = regs::create_reg_universe_systemv(&flags);