[llvm] [TableGen][NFCI] Speed up generating *GenRegisterInfo.inc files on builds with expensive checks. (PR #67340)

[Ivan Kosarev via llvm-commits]

kosarev wrote:

This helped working on https://github.com/llvm/llvm-project/pull/67245.

Note that the Debian expensive-checks bot, <https://lab.llvm.org/buildbot/#/builders/16>, explicitly undefines `_GLIBCXX_DEBUG`, which is a bit unfortunate because when the checks are not disabled, they do catch some problems on the current ToT:
```
Failed Tests (16):
  Clang :: Misc/warning-flags-tree.c
  Clang :: Misc/warning-wall.c
  LLVM :: CodeGen/PowerPC/fp-strict.ll
  LLVM :: CodeGen/PowerPC/pr55463.ll
  LLVM :: CodeGen/PowerPC/register-pressure.ll
  LLVM :: CodeGen/PowerPC/spe.ll
  LLVM :: CodeGen/X86/code_placement_ext_tsp.ll
  LLVM :: CodeGen/X86/code_placement_ext_tsp_large.ll
  LLVM :: Transforms/Inline/nested-inline.ll
  LLVM :: Transforms/SampleProfile/csspgo-import-list.ll
  LLVM-Unit :: ADT/./ADTTests/14/47
  LLVM-Unit :: ADT/./ADTTests/22/47
  LLVM-Unit :: ADT/./ADTTests/30/47
  LLVM-Unit :: ADT/./ADTTests/38/47
  LLVM-Unit :: ADT/./ADTTests/46/47
  LLVM-Unit :: ADT/./ADTTests/7/47
```

#67337 should fix the ADT failures.

As a side note, I don't know why AMDGPU is that much different in regards to the number of (generated) register classes; it might be worth looking into that at some point.

Tagging Galina @gkistanova for awarness.

https://github.com/llvm/llvm-project/pull/67340