[X86] Use target independent ZERO_EXTEND/SIGN_EXTEND nodes were possible in LowerMULH
[X86] Use target independent ZERO_EXTEND/SIGN_EXTEND nodes were possible in LowerMULH

We aren't do any in register extends here so we should be able to just the target independent nodes directly and allow them to be lowered as necessary.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@314447 91177308-0d34-0410-b5e6-96231b3b80d8

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