[InstCombine] Add KnownBits consistency assertion behind option (NFC)

I'm occasionally using this to find cases where computeKnownBits()
and SimplifyDemandedBits() went out of sync.

This option is not enabled by default (even under EXPENSIVE_CHECKS)
because it has a number of known failures in our tests. The reason
for this failures is that computeKnownBits() performs recursive
queries using the original context instruction, while
SimplifyDemandedBits() uses the current instruction. This is
something we can improve, but using the original context wouldn't
always be safe in this context (when non-speculatable instructions
are involved).