[clang] [llvm] Disable pre-link LTO vectorisation and loop unroll for at O2 and O3 optimisation levels (PR #195886)

[Paul Walker via cfe-commits]

paulwalker-arm wrote:

Is it possible to add a controlling command-line option? to allow anybody to test the new configuration out-the-box. Having such an option would remain useful, even if for just the release following its enablement, to allow users to better classify related issues during their toolchain upgrade.

https://github.com/llvm/llvm-project/pull/195886