Fix using array elements in expressions with an array element lval.

When you have an expression like this (extreme example):

a[idx[1]][idx[2]*4 +: 4] <= #(idx[3]) 4'ha;

where a is a reg array and idx is a reg or net array.  The retrieval
of idx[2] was clobbering index register 3, which was set before
evaluating the part offset expression, then used in the %set/av of the
array value.  (likewise for idx[1] and idx[3]])

To avoid this issue, this patch adds and uses a new instruction
%ix/mov which simply copies one indexed register to another.  When
necessary, expressions are first evaluated into temporary registers to
avoid clobbering, then moved in to place before the %*/av instruction.