Emit sub r, #c instead of transforming it to add r, #-c if c fits in 8-bit. This...

Emit sub r, #c instead of transforming it to add r, #-c if c fits in 8-bit. This is a bit of pre-mature optimization. 8-bit variant makes it likely it will be narrowed to a 16-bit instruction.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@78030 91177308-0d34-0410-b5e6-96231b3b80d8