changed naming of blocks / phi-nodes from `vectorial-' to `sierra-' to be
conformant with the naming inside `EmitBranchOnBoolExpr'
removed superfluous instructions to add incoming edges: edges have already been
added by the recursion of `EmitBranchOnBoolExpr', when it reaches the leafs of
the expression tree
removed superfluous and potentially wrong code for `EmitBranchOnBoolExpr's
general fall-through case;
the code seems not to be correct yet: i have to verify that the right code is
emitted depending on the `falseFirst' parameter;
furthermore, invoking `EmitScalarExpr' will cause a crash if the subexpression
is of vectorial type.