[DebugInfo] Fix pr41175 Dead Store Elimination missing debug loc

Bug: https://bugs.llvm.org/show_bug.cgi?id=41175

In the bug test case the DSE pass is shortening the range of memory that a
memset is working on. A getelementptr is generated so that the new
starting address can be passed to memset. This instruction was not given
a DebugLoc.

To fix the bug, copy the DebugLoc from the memset instruction.

Patch by Orlando Cazalet-Hyams!

Differential Revision: https://reviews.llvm.org/D60556

llvm-svn: 358270