compiler: add `intcast_safe` AIR instruction

This instruction is like `intcast`, but includes two safety checks:

* Checks that the int is in range of the destination type
* If the destination type is an exhaustive enum, checks that the int
  is a named enum value

This instruction is locked behind the `safety_checked_instructions`
backend feature; if unsupported, Sema will emit a fallback, as with
other safety-checked instructions.

This instruction is used to add a missing safety check for `@enumFromInt`
truncating bits. This check also has a fallback for backends which do
not yet support `safety_checked_instructions`.

Resolves: #21946

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