Shaders that use the llvm.amdgcn.init.whole.wave intrinsic need to
explicitly preserve the inactive lanes of VGPRs of interest by adding
them as dummy arguments. The code usually looks something like this:
```
define amdgcn_cs_chain void f(active vgpr args..., i32 %inactive.vgpr1, ..., i32 %inactive.vgprN) {
entry:
%c = call i1 @llvm.amdgcn.init.whole.wave()
br i1 %c, label %shader, label %tail
shader:
[...]
tail:
%inactive.vgpr.arg1 = phi i32 [ %inactive.vgpr1, %entry], [poison, %shader]
[...]
; %inactive.vgpr* then get passed into a llvm.amdgcn.cs.chain call
```
Unfortunately, this kind of phi node will get optimized away and the
backend won't be able to figure out that it's ok to use the active lanes
of `%inactive.vgpr*` inside `shader`.
This patch fixes the issue by introducing a llvm.amdgcn.dead intrinsic,
whose result can be used as a PHI operand instead of the poison. This
will be selected to an IMPLICIT_DEF, which the backend can work with.
At the moment, the llvm.amdgcn.dead intrinsic works only on i32 values.
Support for other types can be added later if needed.
611a648327