[Debug] Retain both sets of debug intrinsics in HoistThenElseCodeToIf

When hoisting common code from the "then" and "else" branches of a condition
to before the "if", there is no need to require that debug intrinsics match
before moving them (and merging them).  Instead, we can simply always keep
all debug intrinsics from both sides of the "if".

This fixes PR36410, which describes a problem where as a result of the attempt
to merge debug locations for two debug intrinsics we end up with an invalid
intrinsic, where the scope indicated in the !dbg location no longer matches
the scope of the variable tracked by the intrinsic.

In addition, this has the benefit that we no longer throw away information
that is actually still valid, helping to generate better debug data.

Reviewed By: vsk

Differential Revision: https://reviews.llvm.org/D44312

llvm-svn: 327175