BUG-2199 + EAS-2222: Fix for the FSRT Multiple Definitions for Same Variable Bug

[swathiatgit]

FSRT should be in “static single assignment form”, i.e. there should be exactly one definition of each variable, as this simplifies later analysis. This PR fixes this bug with mutable iteration over the created environment, and ensures that every variable has one definition.

More on the bug: Take this code example below:

function foo(a) {
  let b = a - 20;
  let c = b + 1;
  b = 10;
  return b * 10;
}

The above function currently gives this IR:

Variables:
%0: return value
%1: local definition of b
%2: local definition of c
%3: local definition of b
%4: local definition of b
%5: global ref to a
%6: temporary
%7: global ref to b
%8: temporary
%9: global ref to c
%10: temporary
bb0:
    %6 = %5 - 20
    %7 = %6 
    %8 = %7 + 1
    %9 = %8
    %7 = 10 // second assignment of %7
    _ = 10
    %10 = %7 * 10
    %0 = %10
    return

Note that in the current output of our IR that every “use” of the variable %7 only needs to consider the definition of “%7 = 10”. Thus, we can replace the assignment “%7 = 10” with “%11 = 10” and all following uses of “%7” with the new variable “%11”.

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[jwong101]

Lgtm. Once you rebase/cleanup your commits I'll merge it in.