[AArch64][GlobalISel] Overhaul G_EXTRACT legalization

This simplifies our existing G_EXTRACT rules and adds some test coverage. Mostly
changing this because it should make it easier to improve legalization for
instructions which use G_EXTRACT as part of the legalization process.

This also adds support for legalizing some weird types. Similar to other recent
legalizer changes, this changes the order of widening/clamping.

There was some dead code in our existing rules (e.g. the p0 case would never get
hit), so this knocks those out and makes the types we want to handle explicit.

This also removes some checks which, nowadays, are handled by the
MachineVerifier.

Differential Revision: https://reviews.llvm.org/D107505