Zero masked arithmetic operations

[mazimkhan]

Introduces:

• MaskedMaxOrZero(m, a, b): returns Max(a, b)[i] or zero if m[i] is false
• MaskedAddOrZero(m, a, b): returns a[i] + b[i] or 0 if m[i] is false.
• MaskedSubOrZero(m, a, b): returns a[i] - b[i] or 0 if m[i] is false.
• MaskedMulOrZero( m, a, b): returns a[i] * b[i] or 0 if m[i] is false.
• MaskedDivideOrZero(m, a, b): returns a[i] / b[i] or 0 if m[i] is false.
• MaskedSaturatedAddOrZero(m, a, b): returns a[i] + b[i] saturated to the minimum/maximum representable value, or 0 if m[i]` is false.
• MaskedSaturatedSubOrZero(m, a, b): returns a[i] - b[i] saturated to the minimum/maximum representable value, or 0 if m[i]` is false.
• MaskedMulFixedPoint15OrZero(m, a): returns returns the result of multiplying two Q1.15 fixed-point numbers, or 0 if m[i] is false.
• MaskedMulAddOrZero(m, a, b, c): returns a[i] * b[i] + c[i] or 0 if m[i] is false.
• MaskedNegMulAddOrZero(m, a, b, c): returns -a[i] * b[i] + c[i] or 0 if m[i] is false.
• MaskedWidenMulPairwiseAddOrZero(d, m, a, b): widens a and b to TFromD<D> and computes a[2*i+1]*b[2*i+1] + a[2*i+0]*b[2*i+0], or 0 if m[i] is false.

Testing is included for all operations where both the masking and the underlying operation is tested.