remove switch statement in vector8_shift_{left,right}

In commit ec8719ccbf, I added a switch statement with all expected shift
counts for vector8_shift_{left,right} because AArch64's vshlq_n_u32 and
vshrq_n_u32 require an integer literal.  I discovered that we can use
vshlq_u32 instead for both cases, which avoids the need for the switch
statement and compiles to the same machine code on recent gcc/clang.

-- 
nathan