[mlir][emitc] Restrict types in EmitC (llvm#88391)

Restrict the types which are valid for EmitC operations. Use what is
currently supported by the emitter as restriction. Define a utility
functions for valid types, such that they can be used to restrict the
operations in table gen as well as being available for reuse in dialect
conversions.

Author: TinaAMD

mlir/include/mlir/Dialect/EmitC/IR/EmitC.td

@@ -1242,7 +1242,7 @@ def EmitC_SubscriptOp : EmitC_Op<"subscript", []> {
       EmitC_OpaqueType,
       EmitC_PointerType]>,
     "the value to subscript">:$value,
-    Variadic<AnyType>:$indices);
+    Variadic<EmitCType>:$indices);
   let results = (outs EmitCType:$result);
 
   let builders = [