[X86][SSE] Reuse zeroable element mask instead of regenerating it. NFCI
[X86][SSE] Reuse zeroable element mask instead of regenerating it. NFCI

We are repeatedly calling computeZeroableShuffleElements in many shuffle lowering calls for the same shuffle mask/inputs.

This is a first step towards reusing the zeroable result, initially just for lowerVectorShuffleAsShift calls.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@286037 91177308-0d34-0410-b5e6-96231b3b80d8

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