[GISel]: Add combine for G_FABS to G_FABS

https://reviews.llvm.org/D87554

Patch adds one new GICombinerRule for G_FABS. The combine rule folds G_FABS(G_FABS(X)) to G_FABS(X).
Patch additionally adds new combiner tests for the AArch64 target to test this new combiner rule.

Patch by mkitzan.