Solves unused variable error in generated Global ISel code due to
changes from #140935

This method doesn't use anything from CodeGenTarget, so it seems to
belong in CodeGenRegBank.

Previously PatLeafs could not be imported, causing the following
warnings to be emitted when running tblgen with
`-warn-on-skipped-patterns:`
```
/work/clean/llvm/lib/Target/AArch64/AArch64InstrInfo.td:2631:1: warning: Skipped pattern: Src pattern child has unsupported predicate
def : Pat<(i64 (mul top32Zero:$Rn, top32Zero:$Rm)),
^
```
These changes allow the patterns to now be imported successfully.

Outside of CodeGenRegisters itself, we only want to find existing
SubRegIdxs, not create new ones. Change findSubRegIdx to assert and use
it consistently for this purpose.

- Change `Name`, `SubopNames`, `PrinterMethodName`, and
`EncoderMethodNames` to be stored as StringRef.
- Also changed `CheckComplexPatMatcher::Name` to StringRef as a fallout
from the above.

Verified that all the tablegen generated files within LLVM are
unchanged.

Rename `ListInit::getValues()` to `getElements()` to better match with
other `ListInit` members like `getElement`. Keep `getValues()` for
existing downstream code but mark it deprecated.

- Also eliminate unneeded std::string() around some literal strings.

Use it for RISC-V as a demonstration. Other targets will follow.