Bump LLVM to 813f7c3 (llvm#3873)

This commit bumps the llvm-project to
llvm/llvm-project@813f7c3.

This commit also updates the usage of `APInt` in `unpack-quant-tensor`
pass by explicitly setting the `implicitTrunc` arg to be `True` whose
default value was changed from True to False here
llvm/llvm-project@3494ee9.

Signed-off-by: Vivek Khandelwal <vivekkhandelwal1424@gmail.com>

lib/Dialect/TorchConversion/Transforms/UnpackQuantTensor.cpp

@@ -104,7 +104,8 @@ class UnpackQuantizedMatmulWeights
       char mask = (1 << unpackedBitWidth) - 1;
       for (int b = 0; b < packRatio; b++) {
         newData[i * packRatio + b] =
-            APInt(unpackedBitWidth, (el & mask) >> (unpackedBitWidth * b));
+            APInt(unpackedBitWidth, (el & mask) >> (unpackedBitWidth * b),
+                  /*isSigned=*/false, /*implicitTrunc=*/true);
         mask = mask << unpackedBitWidth;
       }
     }