[SelectionDAG] Support 'bit preserving' floating points bitcasts on computeKnownBits...

[SelectionDAG] Support 'bit preserving' floating points bitcasts on computeKnownBits/ComputeNumSignBits

For cases where we know the floating point representations match the bitcasted integer equivalent, allow bitcasting to these types.

This is especially useful for the X86 floating point compare results which return all/zero bits but as a floating point type.

Differential Revision: https://reviews.llvm.org/D39289

llvm-svn: 316831