cpu: Add a [] operator to the RegClass type.

This will create a RegId with the given index.

Change-Id: I2c3e6859fce80ede5cdb70f85d881b514b6babb2
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/49782
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/cpu/reg_class.hh

@@ -128,6 +128,8 @@ class RegClass
 
     inline iterator begin() const;
     inline iterator end() const;
+
+    inline constexpr RegId operator[](RegIndex idx) const;
 };
 
 /** Register ID: describe an architectural register with its class and index.
@@ -280,6 +282,12 @@ RegClass::end() const
     return RegClassIterator(*this, numRegs());
 }
 
+constexpr RegId
+RegClass::operator[](RegIndex idx) const
+{
+    return RegId(type(), idx);
+}
+
 template <typename ValueType>
 class TypedRegClassOps : public RegClassOps
 {