[FuncSpec] Prevent assertion failure when no store value is found

If the only user of the Alloca argument provided to getPromotableAlloca()
is the same as the Call argument, StoreValue is never set and results
in an assertion failure that isa<> was used on a nullptr when passed into
getCandidateConstant().

This was originally seen when trying to build SPEC 2006 416.gamess using
flang with lto enabled.

Differential Revision: https://reviews.llvm.org/D143457