Introduce new convenience methods for sign extending or

truncating an SDValue (depending on whether the target
type is bigger or smaller than the value's type); or zero
extending or truncating it.  Use it in a few places (this
seems to be a popular operation, but I only modified cases
of it in SelectionDAGBuild).  In particular, the eh_selector
lowering was doing this wrong due to a repeated rather than
inverted test, fixed with this change.

llvm-svn: 84027