[SR-13903] Make `ApplyInst` a `MultipleValueInstruction`

Previous ID	SR-13903
Radar	rdar://problem/71913175
Original Reporter	@dan-zheng
Type	Improvement

Votes	0
Component/s	Compiler
Labels	Improvement, SILOptimizer, StarterBug
Assignee	sachinvas16 (JIRA)
Priority	Medium

md5: 41f0f57e0ecea221993e51084b3f1975

Issue Description:

Forum question with context: https://forums.swift.org/t/make-applyinst-a-multiplevalueinstruction/42294

---

SIL supports instructions that return multiple values (i.e. multiple result {{SILValue}}s).

However, apply instructions don't return multiple values. Instead, they produce a single value, which may have a tuple type. This leads to much avoidable destructuring and restructuring of tuples, which complicates SIL transformations like autodiff:

// example.swift
@_silgen_name("foo")
func foo() -> (Int, Int) {
  (0, 0)
}

@_silgen_name("bar")
func bar() -> (Int, Int) {
  return foo()
}

$ swiftc -emit-silgen example.swift
sil hidden [ossa] @bar : $@convention(thin) () -> (Int, Int) {
bb0:
  %0 = function_ref @foo : $@convention(thin) () -> (Int, Int)
  %1 = apply %0() : $@convention(thin) () -> (Int, Int)
  (%2, %3) = destructure_tuple %1 : $(Int, Int) // this is not good
  %4 = tuple (%2 : $Int, %3 : $Int)
  return %4 : $(Int, Int)
}

---

Instead, we can change ApplyInst to inherit MultipleResultInstruction.

Steps:

1. Change definition: make ApplyInst inherit MultipleValueInstruction.

Fix obvious compilation errors in the compiler codebase. Fix as many tests as possible.

2. Update and fix ApplyInst users: SIL generation + parsing + verification + printing, analyses and transformations, IRGen.