Recommit "[SystemZ] Do not emit VEXTEND or VROUND nodes without vector support."

It seems there were some problem with using a .mir test. For some reason
doing '-stop-before=codegenprepare' and then '-start-before=codegenprepare'
on the output .mir file results in the NoVRegs Property after instruction
selection.

Recommitting the same test as an .ll file instead.

llvm-svn: 354160