SROA may decide that it needs to insert a bitcast and would set it's
insertion point before a PHI. This will create an invalid module
right quick.
Instead, choose the first insertion point in the basic block that holds
our PHI.
This fixes PR20822.
Differential Revision: http://reviews.llvm.org/D5141
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216891 91177308-0d34-0410-b5e6-96231b3b80d8