x64: Add test for a fixed issue

This commit adds a test from #3337 which is an issue that was fixed
in #3506 due to moving `imul` lowering rules to ISLE which fixed the
underlying issue of accidentally not falling through to the necessary
case for general `i64x2.mul` multiplication.

Closes #3337

tests/misc_testsuite/simd/almost-extmul.wast

@@ -0,0 +1,16 @@
+;; regression test from #3337, there's a multiplication that sort of
+;; looks like an extmul and codegen shouldn't pattern match too much
+(module
+  (type (;0;) (func))
+  (func (;0;) (type 0)
+    v128.const i32x4 0x00000000 0x00000000 0x00000000 0x00000000
+    i64x2.extend_low_i32x4_u
+    v128.const i32x4 0x00000000 0x00000000 0x00000000 0x00000000
+    i64x2.mul
+    i32x4.all_true
+    i64.load offset=1 align=1
+    drop
+    unreachable)
+  (func (;1;) (type 0)
+    nop)
+  (memory (;0;) 5613 17832))