[X86] Sign extend the 8-bit immediate when commuting blend instructions to match isel.

Conversion from ConstantSDNode to MachineInstr sign extends immediates from their APInt representation to int64_t.

This commit makes sure we do the same for commuting. The tests changes show how this improves CSE. This issue was made worse by the MachineCSE using commuteInstruction to undo a commute. So we virtually guarantee the sign extend from isel would be lost.

The improved CSE also occurred with r354363, but that was reverted. I'm working to undo the revert, but wanted to get this fix in while it was easy to see the results.

llvm-svn: 354724