Fix bug preventing access to formal parameters in addition to variables

[heksterb]

The code as provided is missing some parts that deal with accessing parameters (as opposed to variables). To demonstrate, run

$ ./build/tools/driver/tinylang --emit-llvm examples/Gcd.mod

and view the generated examples/Gcd.ll. Note specifically the while.cond block does not have instructions for the two assignments to parameters a and b. Before my changes:

while.cond:                                       ; preds = %while.body, %after.if
  %1 = icmp ne i64 %b, 0
  br i1 %1, label %while.body, label %after.while

With my changes:

while.cond:                                       ; preds = %while.body, %after.if
  %1 = phi i64 [ %2, %while.body ], [ %a, %after.if ]
  %2 = phi i64 [ %4, %while.body ], [ %b, %after.if ]
  %3 = icmp ne i64 %2, 0
  br i1 %3, label %while.body, label %after.while

Also verified with a test program that the generated function produces the correct numerical result.

[OrchidRock]

CR Ok!