gallium: add a temporary array register file

gallium: add a temporary array register file

like normal temporaries, but allows to define a number of distinct
arrays, all of which make it explicit that they contain /indexable/
registers.
as a side-effect we're adding support for multi-dimensional destination
registers.
The whole thing looks like this:
DCL TEMPX[0][0..128]  # 0 array with 128 registers

ADD TEMPX[0][0], IN[0], IMM[0]
ADD TEMPX[0][1], IN[0], IMM[0]
ABS OUT[0], TEMPX[0][TEMP[0]]