[s390x] Improve handling of negative i32 constants (#6952)

After https://github.com/bytecodealliance/wasmtime/pull/6850 was merged,
the s390x back-end now always uses iilf instead of lhi to load negative
constants, which is a small code size regression.

Fix the isle predicate to get back lhi whenever possible.

cranelift/codegen/src/isa/s390x/inst.isle

@@ -2989,7 +2989,7 @@
         dst))
 
 ;; 32-bit result type, value fits in i16
-(rule 6 (imm (gpr32_ty ty) (i16_from_u64 n))
+(rule 6 (imm (gpr32_ty ty) (u32_pair _ (i16_from_u32 n)))
       (let ((dst WritableReg (temp_writable_reg ty))
             (_ Unit (emit (MInst.Mov32SImm16 dst n))))
         dst))

cranelift/filetests/filetests/isa/s390x/constants.clif

@@ -189,11 +189,11 @@ block0:
 
 ; VCode:
 ; block0:
-;   iilf %r2, 4294967295
+;   lhi %r2, -1
 ;   br %r14
 ;
 ; Disassembled:
 ; block0: ; offset 0x0
-;   iilf %r2, 0xffffffff
+;   lhi %r2, -1
 ;   br %r14