remove redundant assignments in generated function bodies

[JeffBezanson]

This was causing us to emit a copy+assignment for each argument of a generated function, x=x; y=y; .... It doesn't really matter, except when one of these functions is compiled without optimizations, causing us to access and assign a variable that the front end thought was unused.

[tkelman]

test case?

[JeffBezanson]

I was only able to trigger this with type inference disabled. Even then, it's hard to say what to test, since there isn't really anything wrong with redundant assignment statements. It would be good to run the tests with inference off every now and then, but due to return_type that probably isn't possible.

[StefanKarpinski]

It would be good to run the tests with inference off every now and then, but due to return_type that probably isn't possible.

If the number of tests that require return_type is not too large, they could be fenced off with

if Sys.inferece_enabled
    @test ...
end