[DirectX] Lower llvm.lifetime.* intrinsics to stores when DXIL version is lower than 1.6

[Icohedron]

Fixes #147394

References DXC for the implementation logic:
https://github.com/microsoft/DirectXShaderCompiler/blob/d751c827ed3b61e87fdf57d0f424cb2d7af30cd0/lib/HLSL/DxilPreparePasses.cpp#L693-L699

If DXIL Version < 1.6 then replace lifetime intrinsics with stores

• For validator version >= 1.6, store an undef
• For validator version < 1.6, store zeros

else keep the lifetime intrinsics in the DXIL.

After this PR, the number of DML shaders failing validation due to #146974 is reduced from 157 to 50.

[github-actions]

⚠️ undef deprecator found issues in your code. ⚠️

You can test this locally with the following command:

git diff -U0 --pickaxe-regex -S '([^a-zA-Z0-9#_-]undef[^a-zA-Z0-9_-]|UndefValue::get)' 'HEAD~1' HEAD llvm/test/CodeGen/DirectX/legalize-lifetimes-valver-1.6.ll llvm/lib/Target/DirectX/DXILOpLowering.cpp llvm/test/CodeGen/DirectX/legalize-lifetimes-valver-1.5.ll

The following files introduce new uses of undef:

• llvm/lib/Target/DirectX/DXILOpLowering.cpp
• llvm/test/CodeGen/DirectX/legalize-lifetimes-valver-1.6.ll

Undef is now deprecated and should only be used in the rare cases where no replacement is possible. For example, a load of uninitialized memory yields undef. You should use poison values for placeholders instead.

In tests, avoid using undef and having tests that trigger undefined behavior. If you need an operand with some unimportant value, you can add a new argument to the function and use that instead.

For example, this is considered a bad practice:

define void @fn() {
  ...
  br i1 undef, ...
}

Please use the following instead:

define void @fn(i1 %cond) {
  ...
  br i1 %cond, ...
}

Please refer to the Undefined Behavior Manual for more information.

[Icohedron]

⚠️ undef deprecator found issues in your code. ⚠️

You can test this locally with the following command:
The following files introduce new uses of undef:

• llvm/lib/Target/DirectX/DXILOpLowering.cpp
• llvm/test/CodeGen/DirectX/legalize-lifetimes-valver-1.6.ll

Undef is now deprecated and should only be used in the rare cases where no replacement is possible. For example, a load of uninitialized memory yields undef. You should use poison values for placeholders instead.

In tests, avoid using undef and having tests that trigger undefined behavior. If you need an operand with some unimportant value, you can add a new argument to the function and use that instead.

For example, this is considered a bad practice:

define void @fn() {
  ...
  br i1 undef, ...
}

Please use the following instead:

define void @fn(i1 %cond) {
  ...
  br i1 %cond, ...
}

Please refer to the Undefined Behavior Manual for more information.

Since the undef is just for DXIL, it should be fine to ignore this? There are other uses of UndefValue::get() within DXILOpLowering.cpp and DXIL tests containing undef.

[farzonl]

Since the undef is just for DXIL, it should be fine to ignore this? There are other uses of UndefValue::get() within DXILOpLowering.cpp and DXIL tests containing undef.

This should be fine DXC will always need undef since llvm 3.7 uses it.

[alsepkow]

LGTM. Assuming farzons feedback is addressed.

[Icohedron]

PR #149310 has broken a test implemented by this PR (llvm/test/CodeGen/DirectX/legalize-lifetimes-valver-1.6.ll) due to lifetime intrinsics being used on bitcast instructions.

I will revert this PR and rethink how to implement lifetime legalization for DXIL.