Reverts llvm/llvm-project#121847

Causes compile time regressions and allegedly miscompilation.

Two instances of `PointerUnion` with different active members and null
value compare unequal. Currently, this results in counterintuitive
behavior when using functions from `Casting.h`, e.g.:

```C++
  PointerUnion<int *, float *> U;
  // U = (int *)nullptr;
  dyn_cast<int *>(U); // Aborts
  dyn_cast<float *>(U); // Aborts
  U = (float *)nullptr;
  dyn_cast<int *>(U); // OK
  dyn_cast<float *>(U); // OK
```

`dyn_cast` should abort in all cases because the argument is null.
Currently, it aborts only if the first member is active. This happens
because the partial template specialization of `ValueIsPresent` for
nullable types compares the union with a union constructed from nullptr,
and the two unions compare equal only if their active members are the
same.

This patch changed the specialization of `ValueIsPresent` for nullable
types to make `isPresent()` return false for all possible null values of
a PointerUnion, and fixes two places where the old behavior was
exploited.

Pull Request: https://github.com/llvm/llvm-project/pull/121847

This patch supports GlobalISel for register bank selection for scalable vector
load and store instructions in RISC-V

These cases in particular are  done as a precommit to support
legalization, regbank selection, and instruction selection for extends,
splat vectors, and integer compares in #85938.

This is a precommit for #71514 to use TypeSize instead of unsigned to
avoid crashes when scalable vectors are used.