[HWToLLVM] Don't crash on OOB accesses

[maerhart]

Operations like hw.array_get produce undefined result values, but no immediate UB. OOB loads in LLVM can cause immediate UB, so the lowerings should guard against that.

[fzi-hielscher]

#6949 #6956

I can resurrect that PR if desired. I had the impression there was no real consensus on how to deal with this properly. Blindly running into UB is obviously a bad idea. But I'm personally not a huge fan of just sweeping the nondeterminism under the carpet. IMHO OOB accesses have a fair chance of being indicative of an actual design problem.