nir: add nir_ssa_for_alu_src()

nir: add nir_ssa_for_alu_src()

Using something like:

   numer = nir_ssa_for_src(bld, alu->src[0].src,
                           nir_ssa_alu_instr_src_components(alu, 0));

for alu src's with swizzle, like:

   vec1 ssa_10 = intrinsic load_uniform () () (0, 0)
   vec2 ssa_11 = intrinsic load_uniform () () (1, 0)
   vec2 ssa_2 = udiv ssa_10.xx, ssa_11

ends up turning into something like:

   vec1 ssa_10 = intrinsic load_uniform () () (0, 0)
   vec2 ssa_11 = intrinsic load_uniform () () (1, 0)
   vec2 ssa_13 = imov ssa_10
   ...

because nir_ssa_for_src() ignore's the original nir_alu_src's swizzle.
Instead for alu instructions, nir_src_for_alu_src() should be used to
ensure the original alu src's swizzle doesn't get lost in translation:

   vec1 ssa_10 = intrinsic load_uniform () () (0, 0)
   vec2 ssa_11 = intrinsic load_uniform () () (1, 0)
   vec2 ssa_13 = imov ssa_10.xx
   ...

v2: check for abs/neg, and re-use existing nir_alu_src

Signed-off-by: Rob Clark <robclark@freedesktop.org>
Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>