Avoid creating conditional cleanup blocks that contain only @llvm.lifetime.end calls

When a non-extended temporary object is created in a conditional branch, the
lifetime of that temporary ends outside the conditional (at the end of the
full-expression). If we're inserting lifetime markers, this means we could end
up generating

  if (some_cond) {
    lifetime.start(&tmp);
    Tmp::Tmp(&tmp);
  }
  // ...
  if (some_cond) {
    lifetime.end(&tmp);
  }

... for a full-expression containing a subexpression of the form `some_cond ?
Tmp().x : 0`. This patch moves the lifetime start for such a temporary out of
the conditional branch so that we don't need to generate an additional basic
block to hold the lifetime end marker.

This is disabled if we want precise lifetime markers (for asan's
stack-use-after-scope checks) or of the temporary has a non-trivial destructor
(in which case we'd generate an extra basic block anyway to hold the destructor
call).

Differential Revision: https://reviews.llvm.org/D50286

llvm-svn: 338945