[Truffle] Expected Absolute Value for Fixnum

[bjfish]

Code Example:

puts (-2**63).abs

Expected:

9223372036854775808

Actual

-9223372036854775808

[chrisseaton]

This is caused by FixnumNodes.AbsNode not checking for overflow.

[bjfish]

@chrisseaton I could try to fix this if you haven't started on it

[chrisseaton]

Go for it.