Change how finalizeBundle selects debug location for the BUNDLE instruction

Summary:
Previously a BUNDLE instruction inherited the DebugLoc from the
first instruction in the bundle, even if that DebugLoc had no
DILocation. With this commit this is changed into selecting the
first DebugLoc that has a DILocation, by searching among the
bundled instructions.

The idea is to reduce amount of bundles that are lacking
debug locations.

Reviewers: #debug-info, JDevlieghere

Reviewed By: JDevlieghere

Subscribers: JDevlieghere, mattd, llvm-commits

Differential Revision: https://reviews.llvm.org/D50639

llvm-svn: 340267