i386: Add HImode to VALID_SSE2_REG_MODE

Move explicit Himode handling for SSE2 XMM regnos from
ix86_hard_regno_mode_ok to VALID_SSE2_REG_MODE.

No functional change.

gcc/ChangeLog:

	* config/i386/i386.cc (ix86_hard_regno_mode_ok):
	Remove explicit HImode handling for SSE2 XMM regnos.
	* config/i386/i386.h (VALID_SSE2_REG_MODE): Add HImode.