[CGP] Merge empty case blocks if no extra moves are added.

Summary:
Currently we skip merging when extra moves may be added in the header of switch instead of the case block, if the case block is used as an incoming
block of a PHI. If all the incoming values of the PHIs are non-constants and the destination block is dominated by the switch block then extra moves are likely not added by ISel, so there is no need to skip merging in this case.

Reviewers: efriedma, junbuml, davidxl, hfinkel, qcolombet

Reviewed By: efriedma

Subscribers: dberlin, kuhar, mcrosier, llvm-commits

Differential Revision: https://reviews.llvm.org/D37343

llvm-svn: 316711