[llvm] r219899 - TRE: make TRE a bit more aggressive

[Rafael Espíndola]

> It is not so much the backend, it is also the language spec:
>
> It says "Both markers imply that the callee does not access allocas or
> varargs from the caller." and I think that is what -dse was using in
> pr7272 to break the code even before codegen.

Running

./bin/opt ~/llvm/llvm/test/Transforms/Inline/byval-tail-call.ll -S
-basicaa -inline -tailcallelim -dse

I now get

define void @frob(i32* %x) {
  %x1 = alloca i32, align 4
  %1 = bitcast i32* %x1 to i8*
  call void @llvm.lifetime.start(i64 4, i8* %1)
  tail call void @ext(i32* %x1)
  tail call void @ext(i32* null)
  %2 = bitcast i32* %x1 to i8*
  call void @llvm.lifetime.end(i64 4, i8* %2)
  ret void
}

Note how %x1 is passed uninitialized to ext.

Cheers,
Rafael