switches with a single case reuse temporary names

Normal switches end the current block by generating the result of the switch expression, then executing a 'switch' instruction to branch to another block where the temporaries are bound for the matched variables, finally branching to another block to bind the variables. A switch with a single case will be optimised by ast_cfg into a single block where a) the switch expression result is generated, and b) temporaries are bound for the matched variables.

However, the temporaries in (b) re-use the same names as the temporaries from (a). This is very bad -- we should not be re-using names in SSA code, especially since the second assignments usually have a different type (so the localtable is incorrect).

For example:

type Pair(a, b):
    Pair(a, b)

def switchpair() :: Int:
    switch Pair(1, 2):
        case Pair(x, y):
            pass
    return x

This generates the assembly code:
def switchpair() :: Int:
    var $T:0 :: Int # Also used as Pair(Int, Int)
    var $T:0:1 :: Int # Also used as (Int, Int) -> Pair(Int, Int)
    var $T:0:2 :: Int
    var $T:0:3 :: Int
    Block 0:
        # Preds: []
        $T:0:1 = Pair
        $T:0:2 = 1
        $T:0:3 = 2
        $T:0 = $T:0:1($T:0:2, $T:0:3)
        $T:0 = $T:0.[Pair: 0]
        $T:0:1 = $T:0.[Pair: 1]
        ....

This produces a run-time error: "Interpreter Error: Field reference to something not an ADT."

1. The ast_cfg should terminate with an internal error if it attempts to set a variable in the localtable which already exists.
2. It needs to generate fresh temporary names for the second part of that block.