i386: Use movss to implement V2SImode VEC_PERM.

Perform V2SI vector permutation in the same way as existing V2SF for
TARGET_MMX_WITH_SSE targets. The testcase:

typedef unsigned int v2si __attribute__((vector_size(8)));
v2si foo(v2si x, v2si y) { return (v2si){y[0], x[1]}; }

is currently compiled to (-O2):

foo:
	movdqa  %xmm0, %xmm2
	movdqa  %xmm1, %xmm0
	pshufd  $0xe5, %xmm2, %xmm2
	punpckldq %xmm2, %xmm0
	ret

and with the patched compiler:

foo:
	movss   %xmm1, %xmm0
	ret

The functionality is already tested in gcc.target/i386/vperm-v2si.c

gcc/ChangeLog:

	* config/i386/i386-expand.cc (expand_vec_perm_movs):
	Handle V2SImode for TARGET_MMX_WITH_SSE.
	* config/i386/mmx.md (*mmx_movss_<mode>): Rename from *mmx_movss
	using V2FI mode iterator to handle both V2SI and V2SF modes.