ast_cfg: Case-bound variables have wrong subscript

ast_cfg is tasked with converting the code to SSA, so all variables must be assigned at at most one point in the code.

The unpacking statements for case variables are not SSA, because the same qualified variable name may be used multiple times. This is because the variables are qualified with the index of the block in which the switch statement appears, not the block in which they are actually generated. Consider:

type AOrB:
    A(Int)
    B(Int)
def foo(x :: AOrB) :: Int:
    switch x:
        case A(y):
            return y
        case B(y):
            return y

This generates the code:

    Block 0:
        # Preds: []
        switch x:
            case A: goto 4
            case B: goto 5
    Block 1:
        # Preds: [3, 2]
        PHI $RET:1 [2 - "$RET:2", 3 - "$RET:3"]
        # Null block terminator.
    Block 2:
        # Preds: [4]
        PHI y:0 [4 - "$T:4"] # Note y:0, should be y:2
        $RET:2 = y:0
        goto 1
    Block 3:
        # Preds: [5]
        PHI y:0 [5 - "$T:5"] # Note y:0, should be y:3
        $RET:3 = y:0
        goto 1
    Block 4:
        # Preds: [0]
        $T:4 = x.A(0)
        goto 2
    Block 5:
        # Preds: [0]
        $T:5 = x.B(0)
        goto 3

In the above example, the variable y:0 is assigned at two distinct program points. Fix this by naming the unpacked variables after the block in which they are unpacked.