[flang][hlfir] Avoid expr buffer reuse when end_associate may cycle.

If end_associate may execute more times than the expr value producer,
then it cannot take ownership of the expr buffer. Otherwise, it may
result in double-free errors.
Note that the LIT test exposes a different issue with fir.alloca
inside the do-loop produced for hlfir.elemental. This may cause
out-of-stack conditions in valid Fortran programs that are not expected
to run out of stack. I will create an issue for this.

Reviewed By: tblah

Differential Revision: https://reviews.llvm.org/D155778