IntInf constants updates.

Various updates to treatment of `IntInf` constants in the compiler.  Highlights:
 * Recognize both Big and Small representations of IntInfs.
 * Translate IntInf consts to Big and Small representations in conversion from SSA to RSSA.  This is consistent with the treatment of other IntInf operations in the conversion.  After the conversion, IntInf is no longer treated as a primitive.
 * Remove `initIntInfs` from program initialization.
 * Constant fold `IntInf_toVector` and `WordVector_toIntInf` primitives.