rs6000: Ensure vec_sld shift count in allowable range [PR109082]

As PR109082 shows, some uses of vec_sld in emmintrin.h don't
strictly guarantee the given shift count is in the range
0-15 (inclusive).  This patch is to make the argument
range constraint honored for those uses.

	PR target/109082

gcc/ChangeLog:

	* config/rs6000/emmintrin.h (_mm_bslli_si128): Check __N is not less
	than zero when calling vec_sld.
	(_mm_bsrli_si128): Return __A if __N is zero, check __N is bigger than
	zero when calling vec_sld.
	(_mm_slli_si128): Return __A if _imm5 is zero, check _imm5 is bigger
	than zero when calling vec_sld.

gcc/testsuite/ChangeLog:

	* gcc.target/powerpc/pr109082.c: New test.