[GlobalISel] Handle more types in narrowScalar for eq/ne G_ICMP

Generalize the existing eq/ne case using `extractParts`. The original code only
handled narrowings for types of width 2n->n. This generalization allows for any
type that can be broken down by `extractParts`.

General overview is:

- Loop over each narrow-sized part and do exactly what the 2-register case did.
- Loop over the leftover-sized parts and do the same thing
- Widen the leftover-sized XOR results to the desired narrow size
- OR that all together and then do the comparison against 0 (just like the old
  code)

This shows up a lot when building clang for AArch64 using GlobalISel, so it's
worth fixing. For the sake of simplicity, this doesn't handle the non-eq/ne
case yet.

Also remove the code in this case that notifies the observer; we're just going
to delete MI anyway so talking to the observer shouldn't be necessary.

Differential Revision: https://reviews.llvm.org/D105161

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