AMDGPU/GlobalISel: Use destination register bank in applyMappingLoad

Large loads on target that does not useFlatForGlobal have to be split
in regbankselect. This did not happen in case when destination had vgpr
bank and address had sgpr bank.
Instead of checking if address bank is sgpr check bank of the destination.

Differential Revision: https://reviews.llvm.org/D101992