Better fix for PR27903.

Summary:
Re-enable lifetime-start-on-first-use for stack coloring,
but explicitly disable it for slots with more than one start
or end lifetime marker.

Bug: 27903

Reviewers: wmi, tejohnson, qcolombet, gbiv

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D20739

llvm-svn: 271412