[SimplifyCFG] Drop debug loc in SpeculativelyExecuteBB

Summary:
According to HowToUpdateDebugInfo.rst:

```
Preserving the debug locations of speculated instructions can make
it seem like a condition is true when it's not (or vice versa), which
leads to a confusing single-stepping experience
```

This patch follows the recommendation to drop debug locations on
speculated instructions.

Reviewers: aprantl, davide

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D82420