`ftn` uses -1 as the dynamic index marker for memref

[dk949]

This was updated in xdslproject/xdsl@2583d378e and xdslproject/xdsl@77398b962.

The constant builtin.DYNAMIC_INDEX is now used as the marker and matches the number used upstream.

This causes all memref<?xtype> to be printed as memref<-1xtype>.