[InstCombine] Fix SSE2/AVX2 vector logical shift by constant

This patch fixes the sse2/avx2 vector shift by constant instcombine call to correctly deal with the fact that the shift amount is formed from the entire lower 64-bit and not just the lowest element as it currently assumes.

e.g.

%1 = tail call <4 x i32> @llvm.x86.sse2.psrl.d(<4 x i32> %v, <4 x i32> <i32 15, i32 15, i32 15, i32 15>)

In this case, (V)PSRLD doesn't perform a lshr by 15 but in fact attempts to shift by 64424509455 ((15 << 32) | 15) - giving a zero result.

In addition, this review also recognizes shift-by-zero from a ConstantAggregateZero type (PR23821).

Differential Revision: http://reviews.llvm.org/D11760

llvm-svn: 244341