[SCEV] Support clearing Block/LoopDispositions for a single value.

Extend forgetBlockAndLoopDisposition to allow clearing information for a
single value. This can be useful when only a single value is changed,
e.g. because the instruction is moved.

We also need to clear the cached values for all SCEV users, because they
may depend on the starting value's disposition.

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D134614