[AMDGPU][AsmParser] Simplify getting source locations of operands. (#158323)

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