test: Use -disable-output instead of -o /dev/null for IR tools (#239824)
opt, llvm-as and llvm-dis can skip writing their output with
-disable-output rather than writing to /dev/null. Convert by script,
dropping -o /dev/null where -disable-output was already used. Skip a
few other special cases significant to the output, and XFAILed tests.
This appears to be very slightly faster (at best up to 1%).
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>