During bottom up fast-isel, instructions emitted to materalize registers are at top...

During bottom up fast-isel, instructions emitted to materalize registers are at top of basic block and do not have debug location. This may misguide debugger while entering the basic block and sometimes debugger provides semi useful view of current location to developer by picking up previous known location as current location. Assign a sensible location to the first instruction in a basic block, if it does not have one location derived from source file, so that debugger can provide meaningful user experience to developers in edge cases.
[take 2]

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@135423 91177308-0d34-0410-b5e6-96231b3b80d8