[llvm] [AMPGPU] Emit s_singleuse_vdst instructions when a register is used multiple times in the same instruction. (PR #89601)

[Matt Arsenault via llvm-commits]

================
@@ -83,26 +84,13 @@ class AMDGPUInsertSingleUseVDST : public MachineFunctionPass {
         // instruction to be marked as a single use producer.
         bool AllProducerOperandsAreSingleUse = true;
 
-        for (const auto &Operand : MI.operands()) {
-          if (!Operand.isReg())
-            continue;
-          const auto Reg = Operand.getReg();
-
-          // Count the number of times each register is read.
-          if (Operand.readsReg())
-            for (const MCRegUnit &Unit : TRI->regunits(Reg))
-              RegisterUseCount[Unit]++;
+        // Gather a list of Registers used before updating use counts to avoid
----------------
arsenm wrote:

Anything tracking per-instruction liveness should be split into a helper function. I feel like there's already something to do something similar for you somewhere, but I can't seem to find it. Is this just building a map from regunit to a 0-1-or-2 counter? 

https://github.com/llvm/llvm-project/pull/89601