[llvm] [RISCV][RegAlloc] Add getCSRFirstUseCost for RISC-V (PR #131349)

[Craig Topper via llvm-commits]

================
@@ -2375,10 +2375,12 @@ void RAGreedy::aboutToRemoveInterval(const LiveInterval &LI) {
 }
 
 void RAGreedy::initializeCSRCost() {
-  // We use the larger one out of the command-line option and the value report
-  // by TRI.
+  // We use the command-line option if it is explicitly set, otherwise use the
+  // larger one out of the command-line option and the value reported by TRI.
   CSRCost = BlockFrequency(
-      std::max((unsigned)CSRFirstTimeCost, TRI->getCSRFirstUseCost()));
+      CSRFirstTimeCost.getNumOccurrences()
+          ? CSRFirstTimeCost
+          : std::max((unsigned)CSRFirstTimeCost, TRI->getCSRFirstUseCost()));
----------------
topperc wrote:

If the command line option isn't explicitly set, `CSRFirstTimeCost` will be the default value of 0 so this max will always return `TRI->getCSRFirstUseCost()`

https://github.com/llvm/llvm-project/pull/131349