NVPTX cannot select AtomicLoad with OpenMP and Eigen

[markdewing]

As mentioned at the end of #64017, if Eigen is modified to remove the dynamic stack allocation, another 'cannot select' error results

LLVM ERROR: Cannot select: t47: i16,ch = AtomicLoad<(dereferenceable load acquire (s8) from `ptr addrspacecast (ptr addrspace(1) @_ZGVZN5Eigen8internal20manage_caching_sizesENS_6ActionEPlS2_S2_E12m_cacheSizes to ptr)`, align 8)> t26, t28
  t28: i64 = addrspacecast[1 -> 0] t51
    t51: i64 = NVPTXISD::Wrapper TargetGlobalAddress:i64<ptr addrspace(1) @_ZGVZN5Eigen8internal20manage_caching_sizesENS_6ActionEPlS2_S2_E12m_cacheSizes> 0
      t50: i64 = TargetGlobalAddress<ptr addrspace(1) @_ZGVZN5Eigen8internal20manage_caching_sizesENS_6ActionEPlS2_S2_E12m_cacheSizes> 0
In function: _ZN5Eigen8internal20manage_caching_sizesENS_6ActionEPlS2_S2_

Use the test case from #64017, and modify Eigen to undefine EIGEN_ALLOCA after it is defined in Core/util/Memory.h
The diff is

diff --git a/Eigen/src/Core/util/Memory.h b/Eigen/src/Core/util/Memory.h
index f4217e261..6a8d2e738 100644
--- a/Eigen/src/Core/util/Memory.h
+++ b/Eigen/src/Core/util/Memory.h
@@ -680,6 +680,8 @@ template<typename T> EIGEN_DEVICE_FUNC T* smart_move(T* start, T* end, T* target
   #undef EIGEN_ALLOCA
 #endif
 
+#undef EIGEN_ALLOCA
+
 // This helper class construct the allocated memory, and takes care of destructing and freeing the handled data
 // at destruction time. In practice this helper class is mainly useful to avoid memory leak in case of exceptions.
 template<typename T> class aligned_stack_memory_handler : noncopyable