[clang] [Driver] Don't default to -mrelax-all for non-RISCV -O0 (PR #90013)

[Nikita Popov via cfe-commits]

nikic wrote:

FYI I'm seeing about 0.6% compile-time regressions for `O0` test-suite builds with this change (https://llvm-compile-time-tracker.com/compare.php?from=ef2ca97f48f1aee1483f0c29de5ba52979bec454&to=18376810f359dbd39d2a0aa0ddfc0f7f50eac199&stat=instructions:u). Though there is also a 4.5% reduction in text size (https://llvm-compile-time-tracker.com/compare.php?from=ef2ca97f48f1aee1483f0c29de5ba52979bec454&to=18376810f359dbd39d2a0aa0ddfc0f7f50eac199&stat=size-text) so maybe the tradeoff is reasonable even for `O0`.

https://github.com/llvm/llvm-project/pull/90013