sse2: fix _mm_bslli_si128 AltiVec implementation on XL C/C++
sse2: fix _mm_bslli_si128 AltiVec implementation on XL C/C++

Even if the code can never be called, on XL C/C++ the third argument
to vec_sld must be in [0, 15].  The fix is a bit silly, but since
we're dealing with compile-time constants it shouldn't be a performance
issue.

nemequ/simde,nemequ/simde,nemequ/simde