[Tablegen] Bugfix and refactor VarLenCodeEmitter HwModes. (#68795)

VarLenCodeEmitterGen produced code that did not compile if using
alternative encoding in different HwModes. It's not possbile to assign

    unsigned **Index = Index_<mode>[][2] = { ... };

As a fix, Index and InstBits where removed in favor of mode specific
getInstBits_<mode> functions since this is the only place the arrays are
accessed.

Handling of HwModes is now concentrated to the VarLenCodeEmitterGen::run
method reducing the overall amount of code and enabling other types of
alternative encodings not related to HwModes.

Added a test for VarLenCodeEmitterGen HwModes.

Make sure that HwModes are supported in the same way they are supported
for the standard CodeEmitter. It should be possible to define
instructions with universal encoding across modes, distinct encodings
for each mode or only define encodings for some modes.

Fixed indentation in generated code.