XCore target: fix bug in XCoreLowerThreadLocal.cpp

[Robert Lytton]

Hi Rafael,

> The issues is just that the return value of the invoke is made
> available at the edge, so if you want to put something in between an
> invoke and a phi use, you have to split the edge.

I understand your concern and the answer is that we don't put anything between the invoke & phi.
If we have a constant-expression in the phi, this will be lowered into an expression before the invoke (in front of the terminal instruction of the calling block).
If we have a constant-expression in the invoke (and all other instructions), it will be lowered into an expression before the invoke.

Thank you for explaining.

Robert