[llvm] [utils] Add a script to update regclass numbers in MIR tests (PR #142761)

[Jay Foad via llvm-commits]

jayfoad wrote:

> There aren't so many MIR tests that contain this kind of comment (I counted only 112 *.mir and 87 *.ll). Why don't just use `update_mir_test_checks.py`?

This is a different kind of update. It is not just updating CHECK lines, it is also updating the source lines, like (example from #142359):
```diff
-    INLINEASM &"; def $0", 1 /* sideeffect attdialect */, 1966090 /* regdef:VGPR_32 */, def $vgpr0
+    INLINEASM &"; def $0", 1 /* sideeffect attdialect */, 2228234 /* regdef:VGPR_32 */, def $vgpr0
```

https://github.com/llvm/llvm-project/pull/142761