Revert r323472 "[Debug] Add dbg.value intrinsics for PHIs created during LCSSA."

This broke the Chromium build; see PR36238.

> This patch is an enhancement to propagate dbg.value information when
> Phis are created on behalf of LCSSA.  I noticed a case where a value
> carried across a loop was reported as <optimized out>.
>
> Specifically this case:
>
>   int bar(int x, int y) {
>     return x + y;
>   }
>
>   int foo(int size) {
>     int val = 0;
>     for (int i = 0; i < size; ++i) {
>       val = bar(val, i);  // Both val and i are correct
>     }
>     return val; // <optimized out>
>   }
>
> In the above case, after all of the interesting computation completes
> our value is reported as "optimized out." This change will add a
> dbg.value to correct this.
>
> This patch also moves the dbg.value insertion routine from
> LoopRotation.cpp into Local.cpp, so that we can share it in both places
> (LoopRotation and LCSSA).
>
> Patch by Matt Davis!
>
> Differential Revision: https://reviews.llvm.org/D42551

llvm-svn: 324247