The unit test for hasUniqueRepresentation asserted that a vector of
length 3 would not have a unique representation. This would be true if
it were lowered to ABI size 8 instead of 6. However lowering it to ABI
size 6 is perfectly valid depending on the target.
This commit also simplifies the logic for hasUniqueRepresentation of
integers.
7cfa97aa4e