Implement a little hack for parity with GCC on crafty. This speeds up

186.crafty by about 16% (from 15.109s to 13.045s) on my system.

This turns allocas with unions/casts into scalars.  For example crafty has
something like this:

    union doub {
      unsigned short i[4];
      long long d;
    };
int f(long long a) {
  return ((union doub){.d=a}).i[1];
}

Instead of generating loads and stores to an alloca, we now promote the
whole thing to a scalar long value.

This implements: Transforms/ScalarRepl/AggregatePromote.ll

llvm-svn: 24667