[SimplifyCFG] Use make_early_inc_range() while deleting instructions

We are deleting `phi` nodes within the for loop, so this makes sure we
increment the iterator before we delete the instruction pointed by the
iterator.

This started to break in
https://github.com/llvm/llvm-project/commit/a0be08164622bf938855ff5d19dd8e9d0c96b9b3.

Reviewed By: dschuff, lebedev.ri

Differential Revision: https://reviews.llvm.org/D103181