StgToJS: Simplify ExprInline constructor of ExprResult

Its payload was used only for a small optimization in genAlts,
avoiding a few assignments for programs of this form:

  case NormalDataCon arg1 arg2 of x { NormalDataCon x1 x2 -> ... ; }

But when compiling with optimizations, this sort of code is
generally eliminated by case-of-known-constructor in Core-to-Core.
So it doesn't seem worth tracking and cleaning up again in StgToJS.