[X86] Remove unnecessary TODO.
[X86] Remove unnecessary TODO.

When I wrote it I thought we were missing a potential optimization for KNL. But investigating further shows that for KNL we still do the optimal thing by widening to v4f32 and then using special isel patterns to widen again to zmm a register.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@320862 91177308-0d34-0410-b5e6-96231b3b80d8

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