[TableGen] Add a new `encoder` directive into VarLenCodeEmitterGen

The new encoder directive can be used to specify custom encoder for a
single operand or slice. This is different from the EncoderMethod field
within an Operand, which affects every operands in the target.

In addition, this patch also changes the function signature of the
encoder method -- a new argument, InsertPost, is added to both the
default one (i.e. getMachineValue) and the custom one. This argument
provides the bit position where the operand will eventually be inserted.

Differential Revision: https://reviews.llvm.org/D119100