[DirectX][DXIL] Distinguish return type for overload type resolution. (#85646)

Return type of DXIL Ops may be different from valid overload type of the
parameters, if any. Such DXIL Ops are correctly represented in DXIL.td.
However, DXILEmitter assumes the return type to be the same as parameter
overload type, if one exists. This results in generation in incorrect
overload index value in DXILOperation.inc for the DXIL Op and incorrect
DXIL operation function call in DXILOpLowering pass.

This change distinguishes return types correctly from parameter overload
types in DXILEmitter backend to handle such DXIL ops.

Add specification for DXIL Op `isinf` and corresponding tests to verify
the above change.

Fixes issue #85125