[llvm] [GlobalIsel] Combine G_ADD and G_SUB with constants (PR #97771)

[Thorsten Schütt via llvm-commits]

tschuett wrote:

> > Firstly, please update the coding style section of the Combiner.rst with the outcome of the discussion.
> 
> This isn't specific to the combiner, it's a general principle.

Fine. I thought something like, e.g., :
Methods for getting a G_CONSTANT from a register in decreasing priority:
* getIConstantFromReg
* getIConstantVRegVal
* getIConstantVRegValWithLookThrough() 

https://github.com/llvm/llvm-project/pull/97771