[llvm] [RISCV][WIP] Let RA do the CSR saves. (PR #90819)

[Mikhail Gudim via llvm-commits]

================
@@ -262,15 +262,21 @@ namespace {
     void HoistOutOfLoop(MachineDomTreeNode *HeaderN, MachineLoop *CurLoop,
                         MachineBasicBlock *CurPreheader);
 
-    void InitRegPressure(MachineBasicBlock *BB);
+    void InitRegPressure(MachineBasicBlock *BB, const MachineLoop *Loop);
----------------
mgudim wrote:

> Can you also help me understand why this patch (without this machine-licm change) leads to longer live ranges?

because on entry to the function we copy each CSR into a virtual register which will be live for the entire function. 

https://github.com/llvm/llvm-project/pull/90819