[IndVarSimplify] Clear block and loop dispositions after moving instr.

Moving an instruction can invalidate the cached block dispositions of
the corresponding SCEV. Invalidate the cached dispositions.

Also fixes a copy-paste error in forgetBlockAndLoopDispositions where
the start expression S was removed from BlockDispositions in the loop
but not the current values. This was also exposed by the new test case.

Fixes #58439.