Synthesize phi arguments in incoming block

New values were always synthesized in the block of the instruction
that needed them. This is incorrect for PHI node whose' value must be
defined in the respective incoming block. This patch temporarily moves
the builder's insert point to the incoming block while synthesizing phi
node arguments. 

This fixes PR25241 (http://llvm.org/bugs/show_bug.cgi?id=25241)

llvm-svn: 250693