Drive-by fix for a doxygen comment in MachineInstr.h.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@188467 91177308-0d34-0410-b5e6-96231b3b80d8
Constify the function parameters.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@188469 91177308-0d34-0410-b5e6-96231b3b80d8