[FuncSpec] Promote stack values before specialization.

After each iteration of the function specializer, constant stack values
are promoted to constant globals in order to enable recursive function
specialization. This should also be done once before running the
specializer. Enables specialization of _QMbrute_forcePdigits_2 from
SPEC2017:548.exchange2_r.

Differential Revision: https://reviews.llvm.org/D152799