Debug Info: Teach LdStHasDebugValue() (Local.cpp) about DIExpressions.

This function is used to check whether a dbg.value intrinsic has already
been inserted, but without comparing the DIExpression, it would erroneously
fire on split aggregates and only the first scalar would survive.

Found via http://reviews.llvm.org/D16867.
<rdar://problem/24456528>

llvm-svn: 261145