[llvm] [X86] Resolve FIXME: Add FPCW as a rounding control register (PR #82452)

[Phoebe Wang via llvm-commits]

phoebewang wrote:

> In tests, this seems to only cause the wait instruction to disappear, which I do not see as necessary in these particular cases.

I think the wait instructions are intended. The affected tests are all for constrained intrinsics which care the precise exception. Per to SDM
> Synchronization problems occur in the time between the moment when the exception is signaled and when it is
actually handled. Because of concurrent execution, integer or system instructions can be executed during this
time. 

We don't know what instructions a function call may execute, so should emit `wait` for conservative.

https://github.com/llvm/llvm-project/pull/82452