that is, extract the machine-native representation.

Nevertheless, all of these tests become redundant if a second access
to the variable is dominated by the first. In that case, partial
redundancy elimination will eliminate all five of these steps and
simply use the pure value in the arithmetic operation. The partial
redundancy elimination is based largely on the value-based algorithms
developed in [][SIMP96][]
developed in [SIMP96][]

We therefore assume here that we will take the brutally simple
approach of generating code that:

  + whenever a local variable (whether potentially aliased or not) is
    loaded, generate a `moveFromCallFrame` instruction transferring a
	Tcl value into an SSA value.