[llvm] Allow Specifying SGMasks for Inline Asm (PR #155491)

[Patrick Simmons via llvm-commits]

linuxrocks123 wrote:

@arsenm, I don't think we're giving users sufficient alternative methods of controlling code generation such that they would be expected not to see a need for using inline assembly as a tool to improve performance.

Instead of this, we could add `__builtin_sched_group_barrier_override_sgmask()` with the semantics that a nonzero argument will override the sgmask for any instructions found prior to it.  A call with 0 could clear the custom mask.  That way, we're both giving users more control and not directly adding any code to support inline assembly.

https://github.com/llvm/llvm-project/pull/155491