Try to fix UNRESOLVED: LLVM :: CodeGen/AArch64/arm64-regress-opt-cmp.s on bots.

This test used to write a .s file until r266971 fixed that.  But on most bots,
the .s file still exists.  Add an rm statement to clean up the bots.  In a few
days, this statement can go away again.

llvm-svn: 267095