amd64 vmx: micro-optimize vmlaunch failure path

Eliminate two unneeded jumps. One is the jmp to the next instruction,
where there is no requrement that vmlaunch is followed by jmp. Another
one conditionally sets %r11d value, and can be replaced by cmovcc.

Reviewed by: markj
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D52136