[analyzer] Discard malloc-overflow bug-report when a known size is malloc'ed.

This patch ignores malloc-overflow bug in two cases:
Case1:
x = a/b; where n < b
malloc (x*n); Then x*n will not overflow.

Case2:
x = a; // when 'a' is a known value.
malloc (x*n);

Also replaced isa with dyn_cast.

Reject multiplication by zero cases in MallocOverflowSecurityChecker
Currently MallocOverflowSecurityChecker does not catch cases like:
malloc(n * 0 * sizeof(int));

This patch rejects such cases.

Two test cases added. malloc-overflow2.c has an example inspired from a code
in linux kernel where the current checker flags a warning while it should not.

A patch by Aditya Kumar!

Differential Revision: http://reviews.llvm.org/D9924

llvm-svn: 248446