AArch64: always clear kill flags up to last eliminated copy

After r261154, we were only clearing flags if the known-zero register was
originally live-in to the basic block, but we have to do it even if not when
more than one COPY has been eliminated, otherwise the user of the first COPY
may still have <kill> marked.

E.g.

BB#N:
    %X0 = COPY %XZR
    STRXui %X0<kill>, <fi#0>
    %X0 = COPY %XZR
    STRXui %X0<kill>, <fi#1>

We can eliminate both copies, X0 is not live-in, but we must clear the kill on
the first store.

Unfortunately, I've been unable to come up with a non-fragile test for this.
I've only seen it in the wild with regalloc-created spills, and attempts to
reproduce that in a reasonable way run afoul of COPY coalescing. Even volatile
asm clobbers were moved around. Should fix the aarch64 bot though.

llvm-svn: 261175