rs6000: Make _mm_slli_si128 and _mm_bslli_si128 consistent [PR109167]

As PR109167 shows, it's unexpected to have two different
implementation ways for _mm_slli_si128 and _mm_bslli_si128,
as gcc/config/i386/emmintrin.h they should be the same.  So
this patch is to fix it accordingly.

	PR target/109167

gcc/ChangeLog:

	* config/rs6000/emmintrin.h (_mm_bslli_si128): Move the implementation
	from ...
	(_mm_slli_si128): ... here.  Change to call _mm_bslli_si128 directly.

gcc/testsuite/ChangeLog:

	* gcc.target/powerpc/pr109167.c: New test.