[llvm] [RISCV][TII] Add and use new hook to optimize/canonicalize instructions after MachineCopyPropagation (PR #137973)

[Matt Arsenault via llvm-commits]

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@@ -870,6 +870,11 @@ void MachineCopyPropagation::forwardUses(MachineInstr &MI) {
     ++NumCopyForwards;
     Changed = true;
   }
+  // Attempt to canonicalize/optimize the instruction now its arguments have
+  // been mutated.
+  if (TII->optimizeInstruction(MI)) {
+    LLVM_DEBUG(dbgs() << "MCP: After optimizeInstruction: " << MI << "\n");
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arsenm wrote:

```suggestion
    LLVM_DEBUG(dbgs() << "MCP: After optimizeInstruction: " << MI);
```

Redundant newline 

https://github.com/llvm/llvm-project/pull/137973