[LibCallSimplifier] fold memset(malloc(x), 0, x) --> calloc(1, x)

This is a step towards solving PR25892:
https://llvm.org/bugs/show_bug.cgi?id=25892

It won't handle the reported case. As noted by the 'TODO' comments in the patch, 
we need to relax the hasOneUse() constraint and also match patterns that include
memset_chk() and the llvm.memset() intrinsic in addition to memset().

Differential Revision: http://reviews.llvm.org/D16337

llvm-svn: 258816