[llvm] [ConstantFolding] Add ilogb in isMathLibCallNoop (PR #122582)

[Vedant Paranjape via llvm-commits]

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VedantParanjape wrote:

It seems this simplification is happening in EarlyCSE and not instcombine.

```
; *** IR Dump Before EarlyCSEPass on ilogb_const1 ***
define i32 @ilogb_const1() {
  %r = call i32 @ilogb(double -7.000000e+00)
  ret i32 %r
}
; *** IR Dump After EarlyCSEPass on ilogb_const1 ***
define i32 @ilogb_const1() {
  ret i32 2
}
```

https://github.com/llvm/llvm-project/pull/122582