[AArch64][GlobalISel] Promote scalar G_SHL constant shift amounts to s64.
    
    This was supposed to be done in the first place as is currently the case for
    G_ASHR and G_LSHR but was forgotten when the original shift legalization
    overhaul was done last year.
    
    This was exposed because we started falling back on s32 = s32, s64 SHLs
    due to a recent combiner change.
    
    Gives a very minor (0.1%) code size -O0 improvement on consumer-typeset.