nir: Add a function to determine if a source is dynamically uniform

nir: Add a function to determine if a source is dynamically uniform

Adds nir_src_is_dynamically_uniform which returns true if the source
is known to be dynamically uniform. This will be used in a later patch
to add a workaround for cases that only work with dynamically uniform
sources. Note that the function is not definitive, it can return false
negatives (but not false positives). Currently it only detects
constants and uniform accesses. It could easily be extended to include
more cases.

Reviewed-by: Matt Turner <mattst88@gmail.com>