[FuncSpec] Return changed if function is changed by tryToReplaceWithConstant

The may get changed before specialization by RunSCCPSolver. In other
words, the pass may change the function without specialization happens.
Add test and comment to reveal this.
And it may return No Changed if the function get changed by
RunSCCPSolver before the specialization. It looks like a potential bug.

Test Plan: check-all

Reviewed By: https://reviews.llvm.org/D107622

Differential Revision: https://reviews.llvm.org/D107622