bitset encoding tends to have a lot of duplication, for ex. many
instructions with the same encoding modulo the fixed pattern.  Now that
encode_bitset is split out into it's own template, so that we can
capture the result, use a hash table to de-duplicate the bitset encoding
into "snippet" functions so that bitset cases with identical encoding
can re-use the same generated code.

Signed-off-by: Rob Clark <robdclark@chromium.org>