swift234

SILOptimizer: Create non-[fragile] specializations of [fragile] functions where possible

Change the optimizer to only make specializations [fragile] if both the
original callee is [fragile] *and* the caller is [fragile].

Otherwise, the specialized callee might be [fragile] even if it is never
called from a [fragile] function, which inhibits the optimizer from
devirtualizing calls inside the specialization.

This opens up some missed optimization opportunities in the performance
inliner and devirtualization, which currently reject fragile->non-fragile
references:

TEST                                                    | OLD_MIN | NEW_MIN | DELTA (%) | SPEEDUP
---                                                     | ---     | ---     | ---       | ---
DictionaryRemoveOfObjects                               | 38391   | 35859   | -6.6%     | **1.07x**
Hanoi                                                   | 5853    | 5288    | -9.7%     | **1.11x**
Phonebook                                               | 18287   | 14988   | -18.0%    | **1.22x**
SetExclusiveOr_OfObjects                                | 20001   | 15906   | -20.5%    | **1.26x**
SetUnion_OfObjects                                      | 16490   | 12370   | -25.0%    | **1.33x**

Right now, passes other than performance inlining and devirtualization
of class methods are not checking invariants on [fragile] functions
at all, which was incorrect; as part of the work on building the
standard library with -enable-resilience, I added these checks, which
regressed performance with resilience disabled. This patch makes up for
these regressions.

Furthermore, once SIL type lowering is aware of resilience, this will
allow the stack promotion pass to make further optimizations after
specializing [fragile] callees.