Remove 'if()' branches from i64 add and subtract code.
Restore the overflow check to llvm_uadd_with_overflow_i64 to account for the case 0xFFFFFFFFFFFFFFFF + 0x0000000100000001 == 0x0000000100000000.
Improve test/cases/uadd_overflow_64_ta2.ll to check that the tricky addition case 0xFFFFFFFFFFFFFFFF + 0x0000000100000001 is handled correctly.
Remove unneeded >>> 0 in llvm_uadd_with_overflow_i64.
Merge pull request #1088 from juj/branchless_i64_ops

Remove 'if()' branches from i64 add and subtract code.