add fallback test.

Author: MarinPostma

tests/disas/winch/x64/i64x2/mul/mul_fallback.wat

@@ -0,0 +1,40 @@
+;;! target = "x86_64"
+;;! test = "winch"
+;;! flags = [ "-Ccranelift-has-avx" ]
+
+(module
+  (memory 1 1)
+  (func (param v128 v128) (result v128) 
+        (i64x2.mul
+          (local.get 0)
+          (local.get 1)
+          )))
+;; wasm[0]::function[0]:
+;;       pushq   %rbp
+;;       movq    %rsp, %rbp
+;;       movq    8(%rdi), %r11
+;;       movq    0x10(%r11), %r11
+;;       addq    $0x30, %r11
+;;       cmpq    %rsp, %r11
+;;       ja      0x72
+;;   1c: movq    %rdi, %r14
+;;       subq    $0x30, %rsp
+;;       movq    %rdi, 0x28(%rsp)
+;;       movq    %rsi, 0x20(%rsp)
+;;       movdqu  %xmm0, 0x10(%rsp)
+;;       movdqu  %xmm1, (%rsp)
+;;       movdqu  (%rsp), %xmm0
+;;       movdqu  0x10(%rsp), %xmm1
+;;       vpsrlq  $0x20, %xmm1, %xmm15
+;;       vpmuldq %xmm0, %xmm15, %xmm2
+;;       vpsrlq  $0x20, %xmm0, %xmm15
+;;       vpmuludq %xmm1, %xmm15, %xmm15
+;;       vpaddq  %xmm2, %xmm15, %xmm15
+;;       vpsllq  $0x20, %xmm15, %xmm15
+;;       vpmuludq %xmm0, %xmm1, %xmm2
+;;       vpaddq  %xmm2, %xmm15, %xmm1
+;;       movdqa  %xmm1, %xmm0
+;;       addq    $0x30, %rsp
+;;       popq    %rbp
+;;       retq
+;;   72: ud2