Unboxing crash tests; move unboxing helpers to TypesUtils

[mernst]

This PR is for typetools#8329.

• Adds two regression tests that crash on this branch (and on master):

  • framework/tests/all-systems/TypeVarPrimitives.java: compound boolean assignment (x &= t, x |= t, x ^= t) where T extends Boolean. CFGTranslationPhaseOne.visitCompoundAssignment throws "Both arguments to logical operation must be numeric or boolean".
  • framework/tests/all-systems/TypeVarArrayDimension.java: new int[t] where T extends Integer. ValueTreeAnnotator.handleDimensions throws "Unexpected null argument: compareByName(null, @BottomVal)". This test is in its own file so that the crash above does not mask it.

  These tests fail until those two crashes are fixed.

• Moves the unboxing helpers out of CFGTranslationPhaseOne into TypesUtils (boxedPrimitiveForUnboxing, isUnboxable, unboxedTypeOrSelf), and uses them in TreeBuilder.buildPrimValueMethodAccess, TypesUtils.canBeNarrowingPrimitiveConversion, narrowAndBox, conversionRequiresNarrowing, and conditionalExprPromotion.

• Removes duplicated isUnboxable(t) ? unboxedTypeOf(t) : t logic, and renames TreeInfo.isBoxed and local variables named isBoxedPrimitive/isRightBoxed to reflect that they are also true for type variables.

• Erases a type only if it is a type variable or intersection type, and at most once per query, rather than on every assignment and argument conversion.

[msridhar]

@mernst I've invited you as a collaborator on this repo, so you can just push directly to the branch if you like