8325083: jdk/incubator/vector/Double512VectorTests.java crashes in Assembler::vex_prefix_and_encode

Reviewed-by: kvn, sviswanathan

Author: Jatin Bhateja

src/hotspot/cpu/x86/x86.ad

@@ -7235,18 +7235,28 @@ instruct vxor_mem(vec dst, vec src, memory mem) %{
 // --------------------------------- VectorCast --------------------------------------
 
 instruct vcastBtoX(vec dst, vec src) %{
+  predicate(VM_Version::supports_avx512vl() || Matcher::vector_element_basic_type(n) != T_DOUBLE);
   match(Set dst (VectorCastB2X src));
   format %{ "vector_cast_b2x $dst,$src\t!" %}
   ins_encode %{
-    assert(UseAVX > 0, "required");
-
     BasicType to_elem_bt = Matcher::vector_element_basic_type(this);
     int vlen_enc = vector_length_encoding(this);
     __ vconvert_b2x(to_elem_bt, $dst$$XMMRegister, $src$$XMMRegister, vlen_enc);
   %}
   ins_pipe( pipe_slow );
 %}
 
+instruct vcastBtoD(legVec dst, legVec src) %{
+  predicate(!VM_Version::supports_avx512vl() && Matcher::vector_element_basic_type(n) == T_DOUBLE);
+  match(Set dst (VectorCastB2X src));
+  format %{ "vector_cast_b2x $dst,$src\t!" %}
+  ins_encode %{
+    int vlen_enc = vector_length_encoding(this);
+    __ vconvert_b2x(T_DOUBLE, $dst$$XMMRegister, $src$$XMMRegister, vlen_enc);
+  %}
+  ins_pipe( pipe_slow );
+%}
+
 instruct castStoX(vec dst, vec src) %{
   predicate((UseAVX <= 2 || !VM_Version::supports_avx512vlbw()) &&
             Matcher::vector_length(n->in(1)) <= 8 && // src