Fix PR26585 by improving the promotion of DBG_VALUEs to DW_AT_locations.

When a variable is described by a single DBG_VALUE instruction we can
often use a more efficient inline DW_AT_location instead of using a
location list.

This commit makes the heuristic that decides when to apply this
optimization stricter by also verifying that the DBG_VALUE is live at the
entry of the function (instead of just checking that it is valid until
the end of the function).

<rdar://problem/24611008>

llvm-svn: 262247