[FuncSpec]Fix assertion failure when value is not added to solver

This patch improves the fix in D110529 to prevent from crashing on value
with byval attribute that is not added in SCCP solver.

Authored-by:  <sinan.lin@linux.alibaba.com>
Reviewed By: ChuanqiXu

Differential Revision: https://reviews.llvm.org/D126355