[X86][AMX][NFC] Make comparison operators to be complete

The previous D101039 didn't fix the SmallSet insertion issue, due to we
always return false for the comparison between 2 different nonnull BBs.
This patch makes the the comparison to be complete by comparing `MBB`
first, so that we can always get the invariant order by a single
operator.

Author: phoebewang

llvm/lib/Target/X86/X86PreTileConfig.cpp

@@ -71,10 +71,10 @@ struct MIRef {
   }
   bool operator!=(const MIRef &RHS) const { return !(*this == RHS); }
   bool operator<(const MIRef &RHS) const {
-    return (!MBB && RHS.MBB) || (MBB == RHS.MBB && Pos < RHS.Pos);
+    return MBB < RHS.MBB || (MBB == RHS.MBB && Pos < RHS.Pos);
   }
   bool operator>(const MIRef &RHS) const {
-    return (!RHS.MBB && MBB) || (MBB == RHS.MBB && Pos > RHS.Pos);
+    return MBB > RHS.MBB || (MBB == RHS.MBB && Pos > RHS.Pos);
   }
 };