[llvm] [NFC][AMDGPU] Move PhiLoweringHelper and related types into AMDGPU namespace (PR #192586)

[Shilei Tian via llvm-commits]

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@@ -26,6 +26,7 @@
 #define DEBUG_TYPE "amdgpu-global-isel-divergence-lowering"
 
 using namespace llvm;
+using namespace llvm::AMDGPU;
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shiltian wrote:

unrelated change

https://github.com/llvm/llvm-project/pull/192586