[GISel][RegBankSelect] Hide assertion failure from LLT::getScalarSizeInBits [2/14]

Summary:
It looks like RegBankSelect can try to assign a bank based on a
DBG_VALUE instead of ignoring it. This eventually leads to an assert
in AArch64RegisterBankInfo::getInstrMapping because there is some info
missing from the DBG_VALUE MachineOperand (I see: `Assertion failed:
(RawData != 0 && "Invalid Type"), function getScalarSizeInBits`).

I'm not 100% sure it's safe to insert DBG_VALUE instructions right
before RegBankSelect (that's what -debugify-and-strip-all-safe is
doing). Any advice appreciated.

Depends on D78135.

Reviewers: ab, qcolombet, dsanders, aprantl

Subscribers: kristof.beyls, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D78137