[JumpThreading] Fix an incorrect Modified status

This fixes PR47297.

When ProcessBlock() was able to constant fold the terminator's
condition, but not do any more transformations, the function would
return false, which would lead to the JumpThreading pass returning an
incorrect modified status. This patch makes so that ProcessBlock()
returns true in such cases. This will trigger an unnecessary invocation
of ProcessBlock() in such cases, but this should be rare to occur.

This was caught using the check introduced by D80916.

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D87392