[llvm] [InstCombine] Fold `fcmp (fmul X, C1), C2` to `fcmp X, C2/C1` (PR #205003)

[Nikita Terentev via llvm-commits]

Seraphimt wrote:

Hi @VachanVY ! When I tryed to impl this transformation, I consider table possible cases for C1, C2, x, op and with it's help try to check limits of validity the transformation. Example:
```
C1 * x op C2 => x op C3
For C1 = 0, x != inf, x!=Nan, C2 = 0 and op = ordEq : 

0 * x ordEq 0 == true
But x ordEq 0 / 0 == false
```
Implies, if we know that x maybe (not inf, not Nan) value and C1, C2 = +-0 then transform invalid. 

I intended to enable this transform for correct cases only but I overwhelmed by number of cases and postponed due to lack of time. 
I hope this helps you in some way.

https://github.com/llvm/llvm-project/pull/205003