[CallSiteSplitting] Support splitting of blocks with instrs before call.

For basic blocks with instructions between the beginning of the block
and a call we have to duplicate the instructions before the call in all
split blocks and add PHI nodes for uses of the duplicated instructions
after the call.

Currently, the threshold for the number of instructions before a call
is quite low, to keep the impact on binary size low.

Reviewers: junbuml, mcrosier, davidxl, davide

Reviewed By: junbuml

Differential Revision: https://reviews.llvm.org/D41860

llvm-svn: 325001