Add support for using the `!if' operator when initializing variables:

Add support for using the `!if' operator when initializing variables:

  class A<bit a, bits<3> x, bits<3> y> {
    bits<3> z;
    let z = !if(a, x, y);
  }

The variable z will get the value of x when 'a' is 1 and 'y' when a is '0'.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@121666 91177308-0d34-0410-b5e6-96231b3b80d8