This implements the DAG patterns to enable instruction selection for the
LDAP1 and STL1 instructions from FEAT_LRCPC3. The instructions should
match the following combinations:
- Aqcuiring atomic load + vector insert element for LDAP1.
- Vector extract element + releasing atomic store for STL1.
Patterns have also been added to cope with the DAG structure found when
dealing with 1-lane sub-vectors.
The tests for a non-atomic load of a vector don't seem necessary. (I forgot you can't do an atomic load of a vector.)
Maybe worth testing "load atomic i64" followed by a bitcast to <2 x i32>, instead.