Fix to i386_insert_ffrees (#2724, #1944)

Fix to i386_insert_ffrees (#2724, #1944)
The i386 native code generator has to arrange that the FPU stack is
clear on exit from any function that uses the FPU.  Unfortunately it
was getting this wrong (and has been ever since this code was written,
I think): it was looking for basic blocks that used the FPU and adding
the code to clear the FPU stack on any non-local exit from the block.
In fact it should be doing this on a whole-function basis, rather than
individual basic blocks.