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

[Pengcheng Wang 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:

> What if the second fusion instruction is an instruction with 2 register sources instead of 1 register and an immediate. We'll need to know which operand to constrain.

Then we need to handle them case by case via opcode when adding hints.

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