Fix infinite loop while folding OpVectorShuffle (#1722)

When folding an OpVectorShuffle where the first operand is defined by
an OpVectorShuffle, is unused, and is equal to the second, we end up
with an infinite loop.  This is because we think we change the
instruction, but it does not actually change.  So we keep trying to
folding the same instruction.

This commit fixes up that specific issue.  When the operand is unused,
we replace it with Null.