Implement SaddSat and SsubSat for the interpreter

Implemented `SaddSat` and `SsubSat` to add and subtract signed vector
values, saturating at the type boundaries rather than overflowing.

Changed the parser to allow signed `i8` immediates in vectors as part of
this work; fixes #3276.

Copyright (c) 2021, Arm Limited.

cranelift/interpreter/src/step.rs

@@ -503,7 +503,13 @@ where
             Value::add_sat,
             true,
         )?),
-        Opcode::SaddSat => unimplemented!("SaddSat"),
+        Opcode::SaddSat => assign(binary_arith(
+            arg(0)?,
+            arg(1)?,
+            ctrl_ty,
+            Value::add_sat,
+            false,
+        )?),
         Opcode::Isub => binary(Value::sub, arg(0)?, arg(1)?)?,
         Opcode::UsubSat => assign(binary_arith(
             arg(0)?,
@@ -512,7 +518,13 @@ where
             Value::sub_sat,
             true,
         )?),
-        Opcode::SsubSat => unimplemented!("SsubSat"),
+        Opcode::SsubSat => assign(binary_arith(
+            arg(0)?,
+            arg(1)?,
+            ctrl_ty,
+            Value::sub_sat,
+            false,
+        )?),
         Opcode::Ineg => binary(Value::sub, Value::int(0, ctrl_ty)?, arg(0)?)?,
         Opcode::Iabs => unimplemented!("Iabs"),
         Opcode::Imul => binary(Value::mul, arg(0)?, arg(1)?)?,