nir/search: handle explicitly sized sources in match_value

nir/search: handle explicitly sized sources in match_value

Previously, this case was being handled in match_expression prior to
calling match_value.  However, there is really no good reason for this
given that match_value has all of the information it needs.  Also, they
weren't being handled properly in the commutative case and putting it in
match_value gives us that for free.

Reviewed-by: Connor Abbott <cwabbott0@gmail.com>