[VE][NFC] Fix use-after-free in VEInstrInfo

First call getOperand, then erase the MachineInstr. Not the other way
round.

Expected to fix test/CodeGen/VE/VELIntrinsics/lvm.ll

Detected by asan buildbot:

  sanitizer-x86_64-linux-fast
  (https://lab.llvm.org/buildbot/#/builders/5/builds/15384)