[globalisel] Select register bank for DBG_VALUE

The register operand of DBG_VALUE is not selected to a proper register
bank in both AArch64 and X86. This would cause getRegClass crash after
global ISel. After discussion, we think the MIR should assume all
vritual register should be set proper register class after global ISel,
so this patch is to fix the gap of DBG_VALUE for AArch64 and X86.

Differential Revision: https://reviews.llvm.org/D129037