Add definition for compound assignment operators

This patch adds definition for compound-assignment operators to the P4
language specification. These operators provide a shorter syntax for
assigning the result of an arithmetic or bitwise operator. The proposed
definition of compound assignment operators is similar the
C99 specification.

https://en.cppreference.com/w/c/language/operator_assignment

Signed-off-by: Radostin Stoyanov <radostin.stoyanov@eng.ox.ac.uk>

p4-16/spec/P4-16-spec.mdk

@@ -5228,6 +5228,36 @@ types (e.g. `structs`) are copied recursively, and all components
 of `header`s are copied, including "validity" bits. Assignment is
 not defined for `extern` values.
 
+### Compound assignment { #sec-compound-assignment }
+
+Compound-assignment operators provide a shorter syntax for assigning
+the result of an arithmetic or bitwise operator. The compound assignment
+operator expressions have the form `LHS op RHS`, where `op` is one of
+`+=`, `-=`, `*=`, `/=`, `%=`, `&=`, `|=`, `^=`, `<<=`, `>>=`, and `LHS`,
+`RHS` are expressions with arithmetic types. The following table shows
+all P4 assignment operators and their equivalent.
+
+|-------------|----------------------------------|-----------|---------------|
+| Operator    | Operator name                    | Example   | Equivalent of |
++-------------+:--------------------------------:+:---------:+:-------------:+
+| `=`         | basic assignment                 | `a = b`   | NA            |
+| `+=`        | addition assignment              | `a += b`  | `a = a + b`   |
+| `-=`        | subtraction assignment           | `a -= b`  | `a = a - b`   |
+| `*=`        | multiplication assignment        | `a *= b`  | `a = a * b`   |
+| `/=`        | division assignment              | `a /= b`  | `a = a / b`   |
+| `%=`        | modulo assignment                | `a %= b`  | `a = a % b`   |
+| `&=`        | bitwise AND assignment           | `a &= b`  | `a = a & b`   |
+| `|=`        | bitwise OR assignment            | `a |= b`  | `a = a | b`   |
+| `^=`        | bitwise XOR assignment           | `a ^= b`  | `a = a ^ b`   |
+| `<<=`       | bitwise left shift assignment    | `a <<= b` | `a = a << b`  |
+| `>>=`       | bitwise right shift assignment   | `a >>= b` | `a = a >> b`  |
+|-------------|----------------------------------|-----------|---------------|
+
+The behavior of every builtin compound-assignment expression with the form `E1 op= E2`,
+where `E1` is a modifiable lvalue expression and `E2` is an rvalue expression,
+is exactly the same as the behavior of the expression `E1 = E1 op E2`.
+
+
 ## Empty statement { #sec-empty-stmt }
 
 The empty statement, written `;` is a no-op.