simd_scalar_shift_by_immediate: Change UnallocatedEncoding() path in SaturatingShiftLeft to ReservedValue()

Strictly speaking, immh being zero is defined as reserved in the ARMv8
reference manual. This was just an error on my part when introducing the
SQSHL immediate scalar variant.

src/frontend/A64/translate/impl/simd_scalar_shift_by_immediate.cpp

@@ -32,7 +32,7 @@ enum class FloatConversionDirection {
 
 bool SaturatingShiftLeft(TranslatorVisitor& v, Imm<4> immh, Imm<3> immb, Vec Vn, Vec Vd, Signedness sign) {
     if (immh == 0b0000) {
-        return v.UnallocatedEncoding();
+        return v.ReservedValue();
     }
 
     const size_t esize = 8U << Common::HighestSetBit(immh.ZeroExtend());