Avoid CHECK and buffer over/underrun in JF instruction (#1570)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/1570

A corrupt value index could have over/under-run the `values_` array. Now we validate the index at init time.

Fail non-fatally when encountering an unexpected dtype for the JF instruction argument.

Reviewed By: JacobSzwejbka

Differential Revision: D52646902

fbshipit-source-id: 936b03beb4497ea533fa35e283df1fe35051a20f