KVM: x86: Drop guest/user-triggerable asserts on IRR/ISR vectors

KVM: x86: Drop guest/user-triggerable asserts on IRR/ISR vectors

Remove the ASSERT()s in apic_find_highest_i{r,s}r() that exist to detect
illegal vectors (0-15 are reserved and never recognized by the local APIC),
as the asserts, if they were ever to be enabled by #defining DEBUG, can be
trivially triggered from both the guest and from userspace, and ultimately
because the ASSERT()s are useless.

In large part due to lack of emulation for the Error Status Register and
its "delayed" read semantics, KVM doesn't filter out bad IRQs (IPIs or
otherwise) when IRQs are sent or received.  Instead, probably by dumb
luck on KVM's part, KVM effectively ignores pending illegal vectors in
the IRR due vector 0-15 having priority '0', and thus never being higher
priority than PPR.

As for ISR, a misbehaving userspace could stuff illegal vector bits, but
again the end result is mostly benign (aside from userspace likely
breaking the VM), as processing illegal vectors "works" and doesn't cause
functional problems.

Regardless of the safety and correctness of KVM's illegal vector handling,
one thing is for certain: the ASSERT()s have done absolutely nothing to
help detect such issues since they were added 18+ years ago by commit
97222cc83163 ("KVM: Emulate local APIC in kernel").

For all intents and purposes, no functional change intended.

Link: https://patch.msgid.link/20251206004311.479939-3-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>