[SSE2] Minor tidyup of v16i8 SHL lowering. NFC.
[SSE2] Minor tidyup of v16i8 SHL lowering. NFC.

Removed code that was replicating v8i16 'shift + mask' implementation that is done more nicely by making use of LowerScalarImmediateShift


git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@236388 91177308-0d34-0410-b5e6-96231b3b80d8

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