Fix GH-22763: Clear ZREG_TYPE_ONLY flag for in-register vars in zend_jit_snapshot_handler()

[arnaud-lb]

Fixes GH-22763.

The reproducer triggers an assertion in zend_jit_use_reg() because a var has jit->ra[var].ref == IR_NULL but jit->ra[var].flags doesn't contain ZREG_LOAD.

This happens because of the following events:

• An in-register variable is spilled to the VM stack frame by IR (IR_REG_SPILL_SPECIAL)
• zend_jit_snapshot_handler() set the stack descriptor to .flags = ZREG_TYPE_ONLY, .reg = ZREG_NONE
• During a subsequent call to zend_jit_snapshot_handler(), the reg is not IR_REG_SPILL_SPECIAL anymore, so .reg is set but .flags remains ZREG_TYPE_ONLY which is inconsistent
• A side trace inherits from this stack descriptor
• zend_jit_trace_deoptimization() doesn't set jit->ra[var].ref for this var because .flags == ZREG_TYPE_ONLY, which causes the assertion failure later

Fix by removing irrelevant flags when setting reg in zend_jit_snapshot_handler().