target/arm: Implement MVE VCTP

Implement the MVE VCTP insn, which sets the VPR.P0 predicate bits so
as to predicate any element at index Rn or greater is predicated.  As
with VPNOT, this insn itself is predicable and subject to beatwise
execution.

The calculation of the mask is the same as is used to determine
ltpmask in mve_element_mask(), but we precalculate masklen in
generated code to avoid having to have 4 helpers specialized by size.

We put the decode line in with the low-overhead-loop insns in
t32.decode because it's logically part of that collection of insn
patterns, even though it is an MVE only insn.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>

target/arm/t32.decode

@@ -748,5 +748,6 @@ BL               1111 0. .......... 11.1 ............         @branch24
       # This is DLSTP
       DLS        1111 0 0000 0 size:2 rn:4 1110 0000 0000 0001
     }
+    VCTP         1111 0 0000 0 size:2 rn:4 1110 1000 0000 0001
   ]
 }