[Hexagon] Fix wrong order of operands for vmux

Shuffle generation uses vmux to collapse vectors resulting from two
individual shuffles into one. The indexes of the elements selected
from the first operand were indicated by 0xFF in the constant vector
used in the compare instruction, but the compare (veqb) set the bits
corresponding to the 0x00 elements, thus inverting the selection.

Reverse the order of operands to vmux to get the correct output.

llvm-svn: 320516