[FuzzMutate] Add mutator to modify instruction flags.

This patch adds a new InstModificationIRStrategy to mutate flags/options
for instructions. For example, it may add or remove nuw/nsw flags from
add, mul, sub, shl instructions or change the predicate for icmp
instructions.

Subtle changes such as those mentioned above should lead to a more
interesting range of inputs. The presence or absence of overflow flags
can expose subtle bugs, for example.

Reviewed By: bogner

Differential Revision: https://reviews.llvm.org/D94905