[SimplifyCFG] Range reduce switches

If a switch is sparse and all the cases (once sorted) are in arithmetic progression, we can extract the common factor out of the switch and create a dense switch. For example:

    switch (i) {
    case 5: ...
    case 9: ...
    case 13: ...
    case 17: ...
    }

can become:

    if ( (i - 5) % 4 ) goto default;
    switch ((i - 5) / 4) {
    case 0: ...
    case 1: ...
    case 2: ...
    case 3: ...
    }

or even better:

   switch ( ROTR(i - 5, 2) {
   case 0: ...
   case 1: ...
   case 2: ...
   case 3: ...
   }

The division and remainder operations could be costly so we only do this if the factor is a power of two, and emit a right-rotate instead of a divide/remainder sequence. Dense switches can be lowered significantly better than sparse switches and can even be transformed into lookup tables.

llvm-svn: 277325

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