[llvm] [AMDGPU] Enable CSE in the GlobalISel post-legalizer combiner (PR #217258)

[Matt Arsenault via llvm-commits]

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@@ -55,6 +57,26 @@ static cl::opt<bool> EnableNewLegality(
 
 static constexpr unsigned MaxRegisterSize = 1024;
 
+// Register class is part of the CSE profile for every instruction that
+// references a register, so notify observers before changing it.
+static void setRegClassWithObserver(MachineIRBuilder &B,
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arsenm wrote:

I really hate the observer pattern, it's very spammy. Can we instead just create new registers instead of trying to recycle? 

https://github.com/llvm/llvm-project/pull/217258