[NFC] [DAGCombiner] Refactor bitcast folding within fabs/fneg

fabs and fneg share a common transformation:

(fneg (bitconvert x)) -> (bitconvert (xor x sign))
(fabs (bitconvert x)) -> (bitconvert (and x ~sign))

This patch separate the code into a single method.

Reviewed By: spatel

Differential Revision: https://reviews.llvm.org/D86862