[llvm] [AMDGPU] Introduce custom MIR formatting for s_wait_alu (PR #176316)

[Matt Arsenault via llvm-commits]

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@@ -16,10 +16,56 @@
 
 using namespace llvm;
 
+static constexpr const char *SWaitAluImmPrefix = ".";
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arsenm wrote:

Could be char?

https://github.com/llvm/llvm-project/pull/176316