i965: Introduce a MOV_INDIRECT opcode.

The geometry and tessellation control shader stages both read from
multiple URB entries (one per vertex).  The thread payload contains
several URB handles which reference these separate memory segments.

In GLSL, these inputs are represented as per-vertex arrays; the
outermost array index selects which vertex's inputs to read.  This
array index does not necessarily need to be constant.

To handle that, we need to use indirect addressing on GRFs to select
which of the thread payload registers has the appropriate URB handle.
(This is before we can even think about applying the pull model!)

This patch introduces a new opcode which performs a MOV from a
source using VxH indirect addressing (which allows each of the 8
SIMD channels to select distinct data.)

Based on a patch by Jason Ekstrand.

v2: Rename from INDIRECT_THREAD_PAYLOAD_MOV to MOV_INDIRECT; make it
    a bit more generic.  Use regs_read() instead of hacking up the
    register allocator.  (Suggested by Jason Ekstrand.)

v3: Fix regs_read() to be more accurate for small unaligned regions.
    Also rebase on Matt's work.

Signed-off-by: Kenneth Graunke <[email protected]>
Reviewed-by: Jason Ekstrand <[email protected]> [v3]
Reviewed-by: Abdiel Janulgue <[email protected]> [v1]

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