[TableGen] More named sub-operands work.

Commit a538d1f1 first added support for named sub-operands in
CodeEmitterGen. We now add a few more features to that, enabling
further target cleanups.

1. Adds support for handling an EncoderMethod in a sub-operand in
CodeEmitterGen. Previously, the specified encoder of a sub-operand was
ignored, and only the default used.

2. Adds support for sub-operands in DecoderEmitter, along with support
for tied sub-operands.

The changes to the decoder required a few minor tweaks to a few
targets, where existing brokeness was exposed. In order to keep this
patch small, I left FIXMEs which will be addressed in upcoming
patches. (Except MIPS16, since its object file emission/decoding is
totally broken).

Differential Revision: https://reviews.llvm.org/D137653