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: 0ff597fd157e6f4fc38580e8d64ab130330d2411@101420 91177308-0d34-0410-b5e6-96231b3b80d8

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