The patch is to improve the memory footprint of pass GlobalOpt.

Also check in a case to repeat the issue, on which 'opt -globalopt' consumes 1.6GB memory.
The big memory footprint cause is that current GlobalOpt one by one hoists and stores the leaf element constant into the global array, in each iteration, it recreates the global array initializer constant and leave the old initializer alone. This may result in many obsolete constants left.
For example:  we have global array @rom = global [16 x i32] zeroinitializer
After the first element value is hoisted and installed:   @rom = global [16 x i32] [ 1, 0, 0, ... ]
After the second element value is installed:  @rom = global [16 x 32] [ 1, 2, 0, 0, ... ]        // here the previous initializer is obsolete
...
When the transform is done, we have 15 obsolete initializers left useless.

llvm-svn: 169079