llvm/Slicer.h: drop uses of instructions that are being deleted with block
llvm/Slicer.h: drop uses of instructions that are being deleted with block

The instructions may have uses out of the block and this use
still has a reference to the instruction. With LLVM compiled
with assertions, we hit one.

Fixes #101

Signed-off-by: Marek Chalupa <a05df156a69ad59faa9af9b666ef78009594c145@gmail.com>

mchalupa/dg,mchalupa/dg,mchalupa/dg,mchalupa/dg