freedreno/ir3: insert extra move into phi

freedreno/ir3: insert extra move into phi

We had an implicit assumption that the phi src was assigned in it's
source (pred) block leading into the phi.  But this is not true with
NIR, so we can't just ignore the source block specified in the
nir_phi_src.  Insert an extra mov in the source block.  If it is not
required the CP pass will take it back out again.

Fixes:

  ./tests/spec/glsl-1.10/execution/vs-call-in-nested-loop.shader_test
  ./tests/spec/glsl-1.10/execution/vs-inner-loop-modifies-outer-loop-var.shader_test

and probably others.

Signed-off-by: Rob Clark <robclark@freedesktop.org>