Restore isa<Ty>(X) asserts inside cast<Ty>(X)

PLEASE DO NOT REVERT without careful consideration, and preferably prior
discussion.

cast<Ty>(X) is a "checked cast". Its entire purpose is explicitly documented
(https://llvm.org/docs/ProgrammersManual.html#the-isa-cast-and-dyn-cast
templates) as catching bad casts by asserting that the cast is valid.
Unfortunately, in a recent rewrite of our casting infrastructure about three
months back, these asserts got dropped.

This is discussed in more detail on discourse in https://discourse.llvm.org/t/cast-x-is-broken-implications-and-proposal-to-address/63033.

Differential Revision: https://reviews.llvm.org/D127231