[Hexagon] Disallow using the same register for Vy/Vx in vdeal/vshuff

Non-assignment forms of vshuff and vdeal use the first two registers
(Vy, Vx) as both inputs and outputs. It is not valid to use the same
register for both Vy and Vx. The double-write error was not detected
previously because of a special case, which is not actually necessary.

Differential Revision: https://reviews.llvm.org/D142251