i965/fs: Use the LRP instruction for ir_triop_lrp when possible.

i965/fs: Use the LRP instruction for ir_triop_lrp when possible.

v2 [mattst88]:
   - Add BRW_OPCODE_LRP to list of CSE-able expressions.
   - Fix op_var[] array size.
   - Rename arguments to emit_lrp to (x, y, a) to clear confusion.
   - Add LRP function to brw_fs.cpp/.h.
   - Corrected comment about LRP instruction arguments in emit_lrp.
v3 [mattst88]:
   - Duplicate MAD code for LRP instead of using a function pointer.
   - Check for != GRF instead of == IMM in emit_lrp.
   - Lower LRP on gen < 6.

Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
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