incorrect code generation for if_else when expression is used in condition and one of the branches.

[brianguenter]

if an expression is used in condition of if_else and in one of the branches then code gen made that branch return nothing. Example:

julia>  h = make_function([if_else(sqrt(x) < 1, sqrt(x), 0.0)], [x])
RuntimeGeneratedFunction(#=in FastDifferentiation=#, #=using FastDifferentiation=#, :((input_variables,)->begin
          #= c:\Users\seatt\source\FastDifferentiation.jl\src\CodeGeneration.jl:229 =#
          #= c:\Users\seatt\source\FastDifferentiation.jl\src\CodeGeneration.jl:229 =# @inbounds begin
                  #= c:\Users\seatt\source\FastDifferentiation.jl\src\CodeGeneration.jl:230 =#
                  begin
                      result_element_type = promote_type(Float64, eltype(input_variables))
                      result = Array{result_element_type}(undef, (1,))
                      var"##319" = sqrt(input_variables[1])
                      var"##318" = var"##319" < 1
                      var"##317" = if var"##318"
                              #= c:\Users\seatt\source\FastDifferentiation.jl\src\CodeGeneration.jl:58 =#
                              begin #####ERROR here should return var"##319" in the true branch. But because this expression was used in the condition of if_else it was skipped as being already defined.
                              end
                          else
                              #= c:\Users\seatt\source\FastDifferentiation.jl\src\CodeGeneration.jl:60 =#
                              begin
                                  var"##s#320" = 0.0
                              end
                          end
                      result[1] = var"##317"
                      return result
                  end
              end
      end))

This PR fixes this bug.