[IR] Support efficient AssertingVH/PoisoningVH lookup

Currently, there doesn't seem to be any way to look up a Value*
in a map/set indexed by AssertingVH/PoisoningVH, without creating
a value handle -- which is fairly expensive, because it involves
adding the value handle to the use list and immediately removing
it again. Using find_as(Value *) does not work (and is in fact
worse than just using find(Value *)), because it will end up
creating multiple value handles during the lookup itself.

For AssertingVH, address this by simply using DenseMapInfo<T *>
instead of manually implementing something. The AssertingVH<T>
will now get coerced to T*, rather than the other way around.

For PoisoningVH, add extra overloads of getHashValue() and
isEqual() that accept a T* argument.

This allows using find_as(Value *) to perform efficient lookups
in assertion-enabled builds.

Differential Revision: https://reviews.llvm.org/D81793

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