Add a new ConstantAggregateZero class, to fix PR239. This makes zero

Add a new ConstantAggregateZero class, to fix PR239.  This makes zero
initializers for constant structs and arrays take constant space, instead of
space proportinal to the number of elements.  This reduces the memory usage of
the LLVM compiler by hundreds of megabytes when compiling some nasty SPEC95
benchmarks.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@11470 91177308-0d34-0410-b5e6-96231b3b80d8