Avoid sinking machine instructions into a loop.

Avoid sinking machine instructions into a loop.

MachineLoopInfo is already available when MachineSinking runs, so the check is
free.

There is no test case because it would require a critical edge into a loop, and
CodeGenPrepare splits those. This check is just to be extra careful.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@101420 91177308-0d34-0410-b5e6-96231b3b80d8