Switch SDTUnaryArithWithFlags to being modeled as a two-result

Switch SDTUnaryArithWithFlags to being modeled as a two-result
ISD node.  The only change in the generated isel code are comments
like:

<                 // Src: (X86dec_flag:i16 GR16:i16:$src)
---
>                 // Src: (X86dec_flag:i16:i32 GR16:i16:$src)

because now it knows that X86dec_flag returns both an i16 (for the result)
and an i32 (for EFLAGS) in this case.  Wewt.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@99369 91177308-0d34-0410-b5e6-96231b3b80d8