Bug fix "GC relocate is incorrectly tied to the statepoint"

[dbakunevich]

Summary

Make Verifier recognize undef tokens as correct IR

Undef tokens may appear in unreached code as result of RAUW of some optimization,
and it should not be considered as bad IR.

Link to bug: https://github.com/llvm/llvm-project/issues/56267

[mkazantsev]

llvm/lib/CodeGen/SelectionDAG/StatepointLowering.cpp

1202

This needs a test.

llvm/lib/IR/IntrinsicInst.cpp

701

remove

[apilipenko]

llvm/lib/CodeGen/SelectionDAG/StatepointLowering.cpp

173

Assert isa<GCStatepointInst, UndefValue>(Statepoint)

1206

Assert isa<GCStatepointInst, UndefValue>(SI)

1235

Assert isa<GCStatepointInst, UndefValue>(SI)

llvm/lib/IR/Verifier.cpp

5159–5160

isa<GCStatepointInst, UndefValue>(Token)

[dbakunevich]

As part of this task, I could not use the

isa<Type_1,Type_2>(Val)

construct, because there was an error when building llvm.
Throughout llvm, the construction

isa<Type1>(Val) || isa<Type_2>(Val)

is often used. Therefore, I propose to solve the full transition problem with

isa<>() || isa<>()

to

isa<,>()

as a separate NFC patch

[mkazantsev]

llvm/lib/CodeGen/SelectionDAG/StatepointLowering.cpp

173

Not sure if this assert is needed at all, because it will naturally be asserted by cast below.

llvm/lib/IR/IntrinsicInst.cpp

719–724

auto*

721

Remove const_cast

llvm/test/Transforms/InstCombine/gc.relocate-verify.ll

12

unreached

[mkazantsev]

Pls change patch description into smth more verbose, like

Make Verifier recognize undef tokens as correct IR

Undef tokens may appear in unreached code as result of RAUW of some optimization,
and it should not be considered as bad IR.

[mkazantsev]

LGTM w/ description fix.