[TableGen] Implement getNamedOperandIdx with another table lookup. NFC. (#151116)
Use direct table lookup instead of a switch over all opcodes.
In my Release+Asserts build this reduced the code size for
AMDGPU::getNamedOperandIdx from 11422 to 80 bytes, and the total size of
all its tables (including the jump table for the switch) from 243564 to
155020 bytes.