ghc-7.2.2: The GHC API     data Imm Source  An immediate value.
Not all of these are directly representable by the machine.
Things like ImmLit are slurped out and put in a data segment
instead.  strImmLit :: String -> ImmSource  Create a ImmLit
containing this string.  litToImm :: CmmLit -> ImmSource
Convert a CmmLit to an Imm. Narrow to the width: a CmmInt might
be out of range, but we assume that ImmInteger only contains in-
range values. A signed value should be fine here.