[AArch64][GlobalISel] Generalize logic for promoting copies

Generalize the 16-bit FPR to 32-bit GPR logic to work for all cases where
destination size is bigger than source size.

Also fixed CheckCopy() always returning true instead of the result of
isValidCopy().

Differential Revision: https://reviews.llvm.org/D77530

Patch by tambre (Raul Tambre)