TableGen builds up a map of "SubRegIdx -> Subclass" where Subclass is
the largest class where all registers have SubRegIdx as a sub-register.
When SubRegIdx (vis-a-vis the sub-register) is artificial it should
still include it in the map. This map is used in various places,
including in the calculation of the Lanemask of a register class, which
otherwise calculates an incorrect lanemask.
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