[InstCombine] Fold select with signbit idiom into fabs (#76342)

This patch folds:
```
((bitcast X to int) <s 0 ? -X : X) -> fabs(X)
((bitcast X to int) >s -1 ? X : -X) -> fabs(X)
((bitcast X to int) <s 0 ? X : -X) -> -fabs(X)
((bitcast X to int) >s -1 ? -X : X) -> -fabs(X)
```
Alive2: https://alive2.llvm.org/ce/z/rGepow