[x86] Fix EFLAGS copy lowering to correctly handle walking past uses in

[x86] Fix EFLAGS copy lowering to correctly handle walking past uses in
multiple successors where some of the uses end up killing the EFLAGS
register.

There was a bug where rather than skipping to the next basic block
queued up with uses once we saw a kill, we stopped processing the blocks
entirely. =/

Test case produces completely nonsensical code w/o this tiny fix.

This was found testing Speculative Load Hardening and split out of that
work.

Differential Revision: https://reviews.llvm.org/D49211

llvm-svn: 336874