[ARM] GlobalISel: Tighten legalizer tests. NFC

Make sure that all the legalizer tests where the original instruction
needs to be removed check for the removal. We do this by adding
CHECK-NOT lines before and after the replacement sequence. This won't
catch pathological cases where the instruction remains somewhere in the
middle of the instruction sequence that's supposed to replace it, but
hopefully that won't occur in practice (since ideally we'd be setting
the insert point for the new instruction sequence either before or after
the original instruction and not fiddle with it while building the
sequence).

llvm-svn: 307647