MIR Serialization: Reenable one of the MIRParser tests by reverting r239805.

The test 'llvm/test/CodeGen/MIR/machine-function.mir' was disabled on 
x86 msc18 in r239805 as it failed. My commit r240054 have fixed the
problem, so this commit reverts the commit that disabled the test as
it should pass now. 

llvm-svn: 240074