[BDCE] Don't forget uses of root instructions seen before the instruction itself

[BDCE] Don't forget uses of root instructions seen before the instruction itself

When visiting the initial list of "root" instructions (those which must always
be alive), for those that are integer-valued (such as invokes returning an
integer), we mark their bits as (initially) all dead (we might, obviously, find
uses of those bits later, but all bits are assumed dead until proven
otherwise). Don't do so, however, if we're already seen a use of those bits by
another root instruction (such as a store).

Fixes a miscompile of the sanitizer unit tests on x86_64.

Also, add a debug line for visiting the root instructions, and remove a debug
line which tried to print instructions being removed (printing dead
instructions is dangerous, and can sometimes crash).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@229618 91177308-0d34-0410-b5e6-96231b3b80d8