[X86][AVX2] Fix v16i16 SHL lowering (PR27730)

The AVX2 v16i16 shift lowering works by unpacking to 2 x v8i32, performing the shift and then truncating the result.

The unpacking is used to place the values in the upper 16-bits so that we can correctly sign-extend for SRA shifts. Unfortunately we weren't ensuring that the lower 16-bits were zero to ensure that SHL correctly shifts in zero bits.

llvm-svn: 271796