When GVN removes a redundant load, it should not modify the debug location of the dominating load.

In the case of a fully redundant load LI dominated by an equivalent load V, GVN
should always preserve the original debug location of V. Otherwise, we risk to
introduce an incorrect stepping.
If V has debug info, then clearly it should not be modified. If V has a null
debugloc, then it is still potentially incorrect to propagate LI's debugloc
because LI may not post-dominate V.

Differential Revision: https://reviews.llvm.org/D27468

llvm-svn: 288903