[llvm] [GlobalISel] Remove dead VRegs after instruction selection (PR #73892)

[Evgenii Kudriashov via llvm-commits]

e-kud wrote:

> Can you also collect some statistics about this? How many VRegs are cleaned up in such a way?

Yes, it is interesting. I've gathered it on the same command line for `tramp3d-v4.cpp` for time measuring. There are 9618 functions:
```
                          Min     Max      Avg
VRegs before cleaning       0    1490    22.85
VRegs after cleaning        0     660    12.37
VRegs cleaned up            0     830    10.48
```
So, on average we have a little bit less than a half of registers without uses or defs.

https://github.com/llvm/llvm-project/pull/73892