Extract error reporting code emitted by CodeEmitterGen into
MCCodeEmitter static members functions.
Additionally, remove unused ErrorHandling.h header from several files.

* Use streams to avoid dealing with std::string
* Print operand masks in hex
* Make the output more succinct

To avoid passing them to member functions.

Change various `InstBits` tables have an entry only for non-pseudo
target instructions and adjust the indexing into these tables
accordingly.

Some minor refactoring related to this:
- Use early return after handling variable length encodings
- Reduce the scope of anonymous namespace to just the class declaration.

Example reductions in these table sizes for some targets:

```
Target      FirstSupportedOpcode         Reduction in size
AMDGPU                     10813         10813 * 16 = 168KB
RISCV                      12051         12051 * 8 = 94KB
```

I believe it became no-op with the removal of the "positionally encoded
operands" functionality (b87dc356 is the last commit in the series).

There are no changes in the generated files.

raw_ostream is capable of printing unsigned or uint64_t directly.

I suspect this originally existed to avoid a trailing comma from the
previous entry. C++ allows trailing commas in arrays so this isn't
necessary.

Rename `getXYZInstructionsByEnumValue()` to just `getXYZInstructions`
and drop the `ByEnumValue` in the name.