[CodeExtractor] Use 'normal destination' BB as insert point to store invoke results.

Currently CodeExtractor tries to use the next node after an invoke to
place the store for the result of the invoke, if it is an out parameter
of the region. This fails, as the invoke terminates the current BB.
In that case, we can place the store in the 'normal destination' BB, as
the result will only be available in that case.


Reviewers: davidxl, davide, efriedma

Reviewed By: davidxl

Differential Revision: https://reviews.llvm.org/D51037

llvm-svn: 340331