[FuncSpec] Conditional jump or move depends on uninitialised value(s).

I found this bug when performing a two-stage build of clang with
Function Specialization enabled and tuned aggressively. The crash
appears only on release builds.

Fixes https://github.com/llvm/llvm-project/issues/55000.

Before accessing the contents of the ArgInfo iterator inside
SCCPInstVisitor::markArgInFuncSpecialization, we should be
checking that the iterator is valid.

Differential Revision: https://reviews.llvm.org/D124114