[clang] [llvm] [RISCV][MC] Support experimental Zilx extension (PR #209419)

[Pengcheng Wang via cfe-commits]

================
@@ -204,6 +204,13 @@ def FeatureZilsdWordAlign
     : SubtargetFeature<"zilsd-word-align", "AllowZilsdWordAlign", "true",
                        "Allow 4-byte alignment for Zilsd LD/SD instructions">;
 
+def FeatureStdExtZilx
+    : RISCVExperimentalExtension<0, 1, "Index Load for Integer">;
----------------
wangpc-pp wrote:

Now it is `'Zilx' (Indexed Integer Load Instructions)`.

https://github.com/llvm/llvm-project/pull/209419