Build lookup tables for switches (PR884)

This adds a transformation to SimplifyCFG that attemps to turn switch
instructions into loads from lookup tables. It works on switches that
are only used to initialize one or more phi nodes in a common successor
basic block, for example:

  int f(int x) {
    switch (x) {
    case 0: return 5;
    case 1: return 4;
    case 2: return -2;
    case 5: return 7;
    case 6: return 9;
    default: return 42;
  }

This speeds up the code by removing the hard-to-predict jump, and
reduces code size by removing the code for the jump targets.

llvm-svn: 163302