[SelectionDAG] Extend immAll(Ones|Zeros)V to handle ISD::SPLAT_VECTOR

The TableGen immAllOnesV and immAllZerosV helpers implicitly wrapped the
ISD::isBuildVectorAll(Ones|Zeros) helper functions. This was inhibiting
their use for targets such as RISC-V which use ISD::SPLAT_VECTOR. In
particular, RISC-V had to define its own 'vnot' fragment.

In order to extend the scope of these nodes to include support for
ISD::SPLAT_VECTOR, two new ISD predicate functions have been introduced:
ISD::isConstantSplatVectorAll(Ones|Zeros). These effectively supersede
the older "isBuildVector" predicates, which are now simple wrappers for
the new functions. They pass a defaulted boolean toggle which preserves
the old behaviour. It is hoped that in time all call-sites can be ported
to the "isConstantSplatVector" functions.

While the use of ISD::isBuildVectorAll(Ones|Zeros) has not changed, the
behaviour of the TableGen immAll(Ones|Zeros)V **has**. To test the new
functionality, the custom RISC-V TableGen fragment has been removed and
replaced with the built-in 'vnot'. To test their use as pattern-roots, two
splat patterns have been updated accordingly.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D94223