[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.