[llvm] [NFCI][AMDGPU] Try to use PressureDiff to Calculate RegPressure. (PR #94221)

[Matt Arsenault via llvm-commits]

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@@ -116,31 +116,85 @@ void GCNSchedStrategy::initialize(ScheduleDAGMI *DAG) {
                     << ", SGPRExcessLimit = " << SGPRExcessLimit << "\n\n");
 }
 
+static bool canUsePressureDiffs(const SUnit &SU) {
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arsenm wrote:

Should document what this is doing and why it's being done 

https://github.com/llvm/llvm-project/pull/94221