BOUND is pretty slow, and requires an odd start/end pair to be placed in memory. I don't see any reason that it would be better than the usual unsigned comparison + branch that languages with bounds checking tend to use.
Besides, much of the difficulty with memory safety in C and C++ is the existence of pointers (or wrappers around them, like iterators), which do not come with an associated length. Length checking machinery can't help with that problem whether special instructions exist or not.
In short, it's doubtful that the availability of these old instructions would make any difference to the practicality of bounds checking on modern x86 machines whatsoever.
Besides, much of the difficulty with memory safety in C and C++ is the existence of pointers (or wrappers around them, like iterators), which do not come with an associated length. Length checking machinery can't help with that problem whether special instructions exist or not.
In short, it's doubtful that the availability of these old instructions would make any difference to the practicality of bounds checking on modern x86 machines whatsoever.