One optimization in split_no_or is the insertion of a split before
a last matrix line that has only variables. Splitting a matrix there
creates two default environments (instead of one for the non-split
matrix), the first of which often gets specialized away by further
refinement, and the second one jumping directly to the catch-all
case -- this produces better code.

The code would detect this case (all-variable last row) by calling
`group_var` on all the patterns of the row, but the `group_*`
functions assume that their input is already simplified, and only the
first column of the row is simplified.

The present commit fixes it by defining a predicates that does not
assume the pattern is simple, and check that "all its heads" are
a variable. In the future we will refactor this code using
Parmatch.Simple_head.t, and the function invocation may change.

Note: two testcases are changed in tests/basic/patmatch_split_no_or,
the first is the actual optimization and the second is just
stamp-related noise.