[FastISel][X86] Fix INC/DEC optimization (r215230)

I accidentally also used INC/DEC for unsigned arithmetic which doesn't work,
because INC/DEC don't set the required flag which is used for the overflow
check.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@215237 91177308-0d34-0410-b5e6-96231b3b80d8