target/arm/translate-a64: Fix mishandling of size in FCMLA decode

In disas_simd_indexed(), for the case of "complex fp", each indexable
element is a complex pair, so the total size is twice that indicated
in the 'size' field in the encoding. We were trying to do this
"double the size" operation with a left shift by 1, but this is
incorrect because the 'size' field is a MO_8/MO_16/MO_32/MO_64
value, and doubling the size should be done by a simple increment.

This meant we were mishandling FCMLA (by element) of values where
the real and imaginary parts are 32-bit floats, and would incorrectly
UNDEF this encoding. (No other insns take this code path, and for
16-bit floats it happens that 1 << 1 and 1 + 1 are both the same).

Backports commit eaefb97a8b97dbf42c016fe65b68b92f99a346f6 from qemu

qemu/target/arm/translate-a64.c

@@ -12851,7 +12851,7 @@ static void disas_simd_indexed(DisasContext *s, uint32_t insn)
 
     case 2: /* complex fp */
         /* Each indexable element is a complex pair.  */
-        size <<= 1;
+        size += 1;
         switch (size) {
         case MO_32:
             if (h && !is_q) {