i386: Allow MMX register modes in SSE registers

i386: Allow MMX register modes in SSE registers

In 64-bit mode, SSE2 can be used to emulate MMX instructions without
3DNOW.  We can use SSE2 to support MMX register modes.

	PR target/89021
	* config/i386/i386-c.c (ix86_target_macros_internal): Define
	__MMX_WITH_SSE__ for TARGET_MMX_WITH_SSE.
	* config/i386/i386.c (ix86_set_reg_reg_cost): Add support for
	TARGET_MMX_WITH_SSE with VALID_MMX_REG_MODE.
	(ix86_vector_mode_supported_p): Likewise.
	* config/i386/i386.h (TARGET_MMX_WITH_SSE): New.

From-SVN: r271213