[mlir][affine] Make loop tiling default options explicit

[Lewuathe]

Summary

Make default loop tiling options explicit from CLI options. We can also set default value for separate option which is declared implicitly.

[ayzhuang]

LGTM, thanks.

mlir/include/mlir/Dialect/Affine/Passes.td

def AffineLoopInvariantCodeMotion
   let constructor = "mlir::createAffineLoopInvariantCodeMotionPass()";
 }
 
 def AffineLoopTiling : Pass<"affine-loop-tile", "func::FuncOp"> {
   let summary = "Tile affine loop nests";
   let constructor = "mlir::createLoopTilingPass()";
   let options = [
     Option<"cacheSizeInKiB", "cache-size", "uint64_t", /*default=*/"512",
-           "Set size of cache to tile for in KiB">,
-    Option<"separate", "separate", "bool", /*default=*/"",
-           "Separate full and partial tiles">,
+           "Set size of cache to tile for in KiB (default: 512)">,
+    Option<"separate", "separate", "bool", /*default=*/"false",
+           "Separate full and partial tiles (default: false)">,
     Option<"tileSize", "tile-size", "unsigned", /*default=*/"",
            "Use this tile size for all loops">,
     ListOption<"tileSizes", "tile-sizes", "unsigned",
                "List of tile sizes for each perfect nest "
                "(overridden by -tile-size)",
                "llvm::cl::ZeroOrMore">,
   ];
 }