Don't indent single alternative case expressions for STG.

Before

Main.main6 :: GHC.Types.Double -> GHC.Types.Double
[GblId, Arity=1, Caf=NoCafRefs, Str=<S,1*U(U)>m, Unf=OtherCon []] =
    \r [ds_s60w]
        case ds_s60w of {
          GHC.Types.D# x_s60y ->
              case *## [x_s60y x_s60y] of sat_s60z [Occ=Once] {
                __DEFAULT ->
                    case *## [sat_s60z x_s60y] of sat_s60A [Occ=Once] {
                      __DEFAULT ->
                          case *## [sat_s60A x_s60y] of sat_s60B [Occ=Once] {
                            __DEFAULT -> GHC.Types.D# [sat_s60B];
                          };
                    };
              };
        };

After

Main.main6 :: GHC.Types.Double -> GHC.Types.Double
[GblId, Arity=1, Caf=NoCafRefs, Str=<S,1*U(U)>m, Unf=OtherCon []] =
    \r [ds_s5YI]
        case ds_s5YI of {
        GHC.Types.D# x_s5YK ->
        case *## [x_s5YK x_s5YK] of sat_s5YL [Occ=Once] {
        __DEFAULT ->
        case *## [sat_s5YL x_s5YK] of sat_s5YM [Occ=Once] {
        __DEFAULT ->
        case *## [sat_s5YM x_s5YK] of sat_s5YN [Occ=Once] {
        __DEFAULT -> GHC.Types.D# [sat_s5YN];
        };
        };
        };
        };