Correct implementation of -verify-machineinstrs such that it's still...

Correct implementation of -verify-machineinstrs such that it's still overridable for EXPENSIVE_CHECKS

-verify-machineinstrs was implemented as a simple bool. As a result, the
'VerifyMachineCode == cl::BOU_UNSET' used by EXPENSIVE_CHECKS to make it on by
default but possible to disable didn't work as intended. Changed
-verify-machineinstrs to a boolOrDefault to correct this.

llvm-svn: 343696