LowerSwitch: Avoid some undefined behaviour

When a case of INT64_MIN was followed by a case that was greater than
zero, we were overflowing a signed integer here. Since we've sorted
the cases here anyway (and thus currentValue must be greater than
nextValue) it's simple enough to avoid this by using addition rather
than subtraction.

Found by UBSAN on existing tests.

llvm-svn: 240201