agx: Implement unary math ops

Implement nir_op_bitfield_reverse, nir_op_bit_count, and
nir_op_ufind_msb. These map to native instructions.  With appropriate
integer render target and multiple render target support, passes:

   dEQP-GLES31.functional.shaders.builtin_functions.integer.bitfieldreverse.*vertex
   dEQP-GLES31.functional.shaders.builtin_functions.integer.bitfieldreverse.*fragment
   dEQP-GLES31.functional.shaders.builtin_functions.integer.bitcount.*vertex
   dEQP-GLES31.functional.shaders.builtin_functions.integer.bitcount.*fragment
   dEQP-GLES31.functional.shaders.builtin_functions.integer.findLSB.*vertex
   dEQP-GLES31.functional.shaders.builtin_functions.integer.findLSB.*fragment
   dEQP-GLES31.functional.shaders.builtin_functions.integer.findMSB.*vertex
   dEQP-GLES31.functional.shaders.builtin_functions.integer.findMSB.*fragment

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>