[llvm] [AMDGPU] - Add constant folding for s_bitreplicate (PR #72366)

[Jessica Del via llvm-commits]

================
@@ -2422,6 +2423,23 @@ static Constant *ConstantFoldScalarCall1(StringRef Name,
 
       return ConstantFP::get(Ty->getContext(), Val);
     }
+
+    case Intrinsic::amdgcn_s_bitreplicate: {
+      uint64_t Val = Op->getZExtValue();
+      uint64_t ReplicatedVal = 0;
+      uint64_t ReplicatedOnes = 0b11;
+      // Input operand is always b32
+      for (unsigned i = 0; i < 32; ++i, ReplicatedOnes <<= 2, Val >>= 1) {
+        uint64_t Bit = Val & 1;
+
+        if (!Bit)
+          continue;
+
+        ReplicatedVal |= ReplicatedOnes;
+      }
----------------
OutOfCache wrote:

My initial version looked more straightforward like this:

```cpp
uint64_t ReplicatedVal = 0;

for (unsigned i = 0; i < 32; ++i) {
  uint64_t Bit = (Val >> i) & 1;

  if (!Bit)
    continue;

  ReplicatedVal |= (1 << (i * 2));
  ReplicatedVal |= (1 << ((i * 2) + 1);
}
```

But I realized we don't need to shift the values by `i` every iteration, since `i` increases by 1 every time. So instead, I shift the original value by 1 every iteration and shift a mask with two ones (`0b11`) by 2.
The current version might be harder to read or understand. Let me know what you think.

https://github.com/llvm/llvm-project/pull/72366