[ConstantHoisting] Fix bug where constant materialization could insert into EH pad

If the incoming block to a phi node is an EH pad, then we will
materialize into an EH pad, which is not supposed to happen. To fix
this, I added a check to see if incoming block of a phi node is an EH
pad before using it as the insertion point.

Differential Revision: https://reviews.llvm.org/D95019