[BranchFolding] Update debug location along with the update of branch instruction.

Summary:
Currently, BranchFolder drops DebugLoc for branch instructions in some places. For example, for the test code attached, the branch instruction of 'entry' block has a DILocation of

```
!12 = !DILocation(line: 6, column: 3, scope: !11)
```

, but this information is gone when then block is lowered because BranchFolder misses it. This patch is a fix for this issue.

Reviewers: qcolombet, aprantl, craig.topper, MatzeB

Reviewed By: aprantl

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D29902

llvm-svn: 295684