ISLE: Resolve overlap in prelude.isle and x64/inst.isle

[elliottt]

Resolve overlap in the ISLE prelude and the x64 inst module by introducing new types that allow better sharing of extractor resuls, or falling back on priorities.

This PR makes the following changes in overlap counts for the different backends:

branch	x64	aarch64	s390x
main	168	214	446
this	138	212	440

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[jameysharp]

I want to give this one more review pass after my first meeting this morning, but I think this is in great shape and I'm expecting to add my approval to the one Chris already left.