[llvm] [clang-tools-extra] [clang] [compiler-rt] [InferAddressSpaces] Fix constant replace to avoid modifying other functions (PR #70611)

[Wenju He via cfe-commits]

wenju-he wrote:

> I think it would be better if we could eliminate ConstantExpr addrspacecasts from the IR altogether, which would avoid most of the complexity here. I would also somewhat prefer to push this DFS into a helper function, but can live with it inline as-is

thank you for the review. I'll draft another PR to modify convertUsersOfConstantsToInstructions to allow change in a function only, so that DFS is not need here.

https://github.com/llvm/llvm-project/pull/70611