[AMDGPU][AsmParser] Simplify getting source locations of operands.

Remember indexes of MCOperands in MCParsedAsmOperands as we add
them to instructions. Then use the indexes to find locations by
known MCOperands indexes.

Happens to fix some reported locations in tests. NFCI otherwise.

getImmLoc() is to be eliminated as well; there's enough work for
another patch.

Author: kosarev