Remove dead zero store to calloc initialized memory

Optimize the following IR:

%1 = tail call noalias i8* @calloc(i64 1, i64 4)
%2 = bitcast i8* %1 to i32*
; This store is dead and should be removed
store i32 0, i32* %2, align 4

Memory returned by calloc is guaranteed to be zero initialized. If the value being stored is the constant zero (and the store is not otherwise observable across threads), we can delete the store.  If the store is to an out of bounds address, it is undefined and thus also removable.

Reviewed By: nicholas

Differential Revision: http://reviews.llvm.org/D3942