SH: Fix outage caused by recently added 2nd combine pass after reg alloc

I've also confirmed on the CSiBE set that the secondary combine pass is
actually beneficial on SH.  It does result in some code size reductions.

gcc/CHangeLog:
	* config/sh/sh.md (mov_neg_si_t): Allow insn and split after
	register allocation.
	(*treg_noop_move): New insn.