[DAGCombine] Adding a hook to improve the precision of fsqrt if the input is denormal

For now, we will hardcode the result as 0.0 if the input is denormal or 0. That will
have the impact the precision. As the fsqrt added belong to the cold path of the
cmp+branch, it won't impact the performance for normal inputs for PowerPC, but improve
the precision if the input is denormal.

Reviewed By: Spatel

Differential Revision: https://reviews.llvm.org/D80974