[AArch64][GlobalISel] Extend smaller than i32 gpr loads/stores in RegBankSelect. (#175810)
A i8 / i16 load and store is only legal for FPR registers. This patch extends
the types on i8/i16 G_LOADS and G_STORES to i32 using anyext / trunc, so that
selection can be simpler and does not need to handle illegal operations.
This can leave some anyext(trunc) operations that could be removed yet but
should be possible to optimize away.