[GVNHoist] drop debug location according to the debug info guide

According to the LLVM debug info update guide: https://llvm.org/docs/HowToUpdateDebugInfo.html,
"Hoisting identical instructions which appear in several successor
blocks into a predecessor block. In this case there is no single
merged instruction. The rule for dropping locations applies".

Thanks to Yuanbo Li for reporting this.

Reviewed By: dblaikie

Reviewers: sebpop, tejohnson, dblaikie

Differential Revision: https://reviews.llvm.org/D122730