DebugInfo: Simplify constant value emission.

This code looks to have become dead at some time in the past. I tried to
reproduce cases where LLVM would emit constants with dataN, but could
not. Upon inspection it seems the code doesn't do that anymore - the
only time a size is provided by isTypeSigned is when the type is signed,
and in those cases we use sdata. dataN is only used for unsigned types
and isTypeSigned doesn't provide a value for sizeInBits in that case.

Remove the dead cases/size plumbing.