[embedded] Fix class_method devirtualizer to consider specialized VTables

[kubamracek]

The performance devirtualizer can introduce / re-introduce generic functions in embedded Swift mode, after MandatoryPerformanceOptimizations has made sure everything is specialized. See the attached test case (actually it's just an existing test but built with -Osize), which currently triggers an IRGen crash due to a generic function being referenced:

Assertion failed: (!f->isGeneric()), function addLazyFunction, file GenDecl.cpp, line 1567.
...
 #8 0x000000010079ace4 swift::irgen::IRGenerator::addLazyFunction(swift::SILFunction*)
 #9 0x000000010079d118 swift::irgen::IRGenModule::getAddrOfSILFunction(swift::SILFunction*, swift::ForDefinition_t, bool, bool)
#10 0x00000001008ffaac (anonymous namespace)::IRGenSILFunction::visitFunctionRefBaseInst(swift::FunctionRefBaseInst*)
#11 0x00000001008e98c0 (anonymous namespace)::IRGenSILFunction::visitSILBasicBlock(swift::SILBasicBlock*)
#12 0x00000001008e1fa8 (anonymous namespace)::IRGenSILFunction::emitSILFunction()
#13 0x00000001008df6b0 swift::irgen::IRGenModule::emitSILFunction(swift::SILFunction*)
#14 0x0000000100798ba0 swift::irgen::IRGenerator::emitGlobalTopLevel(std::__1::vector<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char>>, std::__1::allocator<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char>>>> const&)

Turns out that the Devirtualizer pass turns this

  %2 = alloc_ref $GenericFooableClass<Int>
  %4 = end_init_let_ref %2 : $GenericFooableClass<Int>
  %7 = class_method %4 : $GenericFooableClass<Int>, #GenericFooableClass.foo : <T> (GenericFooableClass<T>) -> () -> (), $@convention(method) (@guaranteed GenericFooableClass<Int>) -> ()
  %8 = apply %7(%4) : $@convention(method) (@guaranteed GenericFooableClass<Int>) -> ()

into:

  %2 = alloc_ref $GenericFooableClass<Int>
  %4 = end_init_let_ref %2 : $GenericFooableClass<Int>
  // function_ref GenericFooableClass.foo()
  %7 = function_ref @$s4main19GenericFooableClassC3fooyyF : $@convention(method) <τ_0_0> (@guaranteed GenericFooableClass<τ_0_0>) -> ()
  %8 = apply %7<Int>(%4) : $@convention(method) <τ_0_0> (@guaranteed GenericFooableClass<τ_0_0>) -> ()

But it should instead call the specialization (from the specialized VTable), not the generic function (from the regular VTable).

[kubamracek]

@swift-ci please test

[kubamracek]

@swift-ci please test

[kubamracek]

@swift-ci please test Windows platform

[eeckstein]

lgtm!

[kubamracek]

@swift-ci please test Windows platform