s390x/tcg: Implement VECTOR SUBTRACT COMPUTE BORROW INDICATION

Let's keep it simple for now and handle 8/16 bit elements via helpers.
Especially for 8/16, we could come up with some bit tricks.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: David Hildenbrand <david@redhat.com>
diff --git a/target/s390x/insn-data.def b/target/s390x/insn-data.def
index 58a61f4..94de3c9 100644
--- a/target/s390x/insn-data.def
+++ b/target/s390x/insn-data.def
@@ -1176,6 +1176,8 @@
     F(0xe77d, VSRLB,   VRR_c, V,   0, 0, 0, 0, vsrl, 0, IF_VEC)
 /* VECTOR SUBTRACT */
     F(0xe7f7, VS,      VRR_c, V,   0, 0, 0, 0, vs, 0, IF_VEC)
+/* VECTOR SUBTRACT COMPUTE BORROW INDICATION */
+    F(0xe7f5, VSCBI,   VRR_c, V,   0, 0, 0, 0, vscbi, 0, IF_VEC)
 
 #ifndef CONFIG_USER_ONLY
 /* COMPARE AND SWAP AND PURGE */