Simplified intrinsic macro mechanism.

Rationale:
Reduces boiler-plate code in all intrinsics code generators.
Also, the newly introduced "unreachable" macro provides a
static verifier that we do not have unreachable and thus
redundant code in the generators. In fact, this change
exposes that the MIPS32 and MIPS64 rotation intrinsics
(IntegerRotateRight, LongRotateRight, IntegerRotateLeft,
LongRotateLeft) are unreachable, since they are handled
as HIR constructs for all architectures. Thus the code
can be removed.

Change-Id: I0309799a0db580232137ded72bb8a7bbd45440a8