[llvm] [MacroFusion][RISCV] Allocate same register for second instruction of fusible pair (PR #77461)

[Wang Pengcheng via llvm-commits]

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@@ -114,6 +114,7 @@ class MachineInstr
                              // this instruction.
     Unpredictable = 1 << 16, // Instruction with unpredictable condition.
     NoConvergent = 1 << 17,  // Call does not require convergence guarantees.
+    Fusible = 1 << 18,       // Instruction is the second of a fusible pair.
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wangpc-pp wrote:

> Is the point to add a flag to simplify the code which detects fusible patterns? 

Yes!

> I realise that some processors may have a lot of fusions, but is adding a new flag really the right solution?

I think this can be feasible, there are already some values in MIFlag that are not so common, like `NoConvergent` (SIMT only I think), `Unpredictable` (X86 only), etc.

https://github.com/llvm/llvm-project/pull/77461