[SelectionDAG] ComputeNumSignBits of TRUNCATE operations

Add basic ComputeNumSignBits support for TRUNCATE ops for cases where the source's number of sign bits overlaps with the truncated size.

Improves X86 SIGN_EXTEND_IN_REG vector cases which were needlessly sign extending boolean vector results.

Differential Revision: https://reviews.llvm.org/D26851

llvm-svn: 287635