Introduce new convenience methods for sign extending or

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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@84027 91177308-0d34-0410-b5e6-96231b3b80d8