Changes

Summary

Commit a087b78bc4c31be5a82eda62f1e4c7fe1fec14ad by Vedant Kumar Wrong debug info generated at -O2 (-O0 is correct) Instcombiner pass was erasing trivially dead instruction without updating dependent llvm.dbg.value. which was not showing programmer current state of variables while debugging. As a part of this fix I did following, Iterate throught all the users (llvm.dbg) of a instruction which is trivially dead and set each if them undef, Before deleting the instruction. Now user will see optimized out, when try to print those variables. This fixes https://bugs.llvm.org/show_bug.cgi?id=43893 This is my first fix to llvm. Patch by kamlesh kumar! Differential Revision: https://reviews.llvm.org/D69809
	llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
	llvm/test/Transforms/InstCombine/pr43893.ll
	llvm/lib/Transforms/InstCombine/InstCombineInternal.h
	llvm/include/llvm/Transforms/Utils/Local.h
	llvm/test/Transforms/InstCombine/cast-mul-select.ll
	llvm/test/Transforms/InstCombine/debuginfo-dce.ll
	llvm/test/Transforms/InstCombine/stacksave-debuginfo.ll
	llvm/lib/Transforms/Utils/Local.cpp