New pattern guards do not support case splitting

The new pattern guards feature (#3880) does not work with case-splitting:
```agda
open import Agda.Builtin.Nat

test : Set
test with x ← 1 = {!x!}  -- split on x here

-- IS: Produces the following lines and error "Unexpected with patterns"
-- test | zero = ?
-- test | suc x = ?  -- split on x here

-- SHOULD: Produce
-- test with suc x ← 1 = ?
```