Added a rule to replace bitshifts by a constant value to a divide by the corresponding power of two.
Added a check to the simplification to ensure the bitshift is valid.
Simplified the integer overflow checks in the simplification.
Fixed bitshift simplify to handle negative shifts and avoid 32 bit overflow.
Fixed if statement in bitshift simplification. Added tests to check correctness of bitshift simplifies.
Merge pull request #379 from kree-colemcalughlin/simplify_bitshifts

New simplify rule for bitshift operators