[PATCH] Fix VGATHER* operand constraints

Add earlyclobber constaints to prevent input register being allocated as
the output register because, according to Intel spec [1], "If any pair
of the index, mask, or destination registers are the same, this
instruction results a UD fault."

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[1] http://software.intel.com/sites/default/files/319433-014.pdf

llvm-svn: 183327