DFSan's set label function should avoid writing to the shadow memory when the write would not change the value in memory.

When writing a label to shadow memory, don't write if the value is already set to the value being written.  This dramatically reduces real memory consumption in programs with sparse use of labels.

Test Plan: It would be nice to test that unnecessary writes are skipped, but I do not see how a unit test could do this.

Patch by Sam Kerner!

Differential Revision: http://reviews.llvm.org/D4894