ZJIT: Rename send_frame_state and send_state

[k0kubun]

There are three different frame states involved in send HIR:

• state (or exit_state): Pre-send FrameState. This should be used for exits.
• send_frame_state: During-send FrameState. It only strips profiled-nil block arg from state to downgrade send to opt_send_without_block for ISEQ calls. See ZJIT: Optimize send with a nil block to SendWithoutBlockDirect ruby/ruby#16621 for details.
• send_state: Post-send FrameState. *splat, *rest, and **kwrest are resolved.

Especially the last two are a bit hard to distinguish by their names. We should either handle the last two in a single place or rename them to something that we can distinguish more easily.

Discussion: ruby#18983 (comment)