[X86] Enable the MachineVerifier by default

The machine verifier was disabled for x86 by default. There are now only
9 tests failing, compared to what previously was between 20 and 30.

This is a good opportunity to file bugs for all the remaining issues,
then explicitly disable the failing tests and enabling the machine
verifier by default.

This allows us to avoid adding new tests that break the verifier.

PR27481

llvm-svn: 345513