[RISCV] Add DAG combines to transform ADD_VL/SUB_VL into widening add/sub.

[RISCV] Add DAG combines to transform ADD_VL/SUB_VL into widening add/sub.

This adds or reuses ISD opcodes for vadd.wv, vaddu.wv, vadd.vv, vaddu.vv
and a similar set for sub.

I've included support for narrowing scalar splats that have known
sign/zero bits similar to what was done for MUL_VL.

The conversion to vwadd.vv proceeds in two phases. First we'll form
a vwadd.wv by narrowing one of the operands. Then we'll visit the
vwadd.wv to try to narrow the other operand. This turned out to be
simpler than catching all the cases in one step. The forming of of
vwadd.wv can happen for either operand for add, but only the right
hand side for sub since sub isn't commutable.

An interesting quirk is that ADD_VL and VZEXT_VL/VSEXT_VL are formed
during vector op legalization, but VMV_V_X_VL isn't usually formed
until op legalization when BUILD_VECTORS are handled. This leads to
VWADD_W_VL forming in one DAG combine round, and then a later DAG combine
round sees the VMV_V_X_VL and needs to commute the operands to get the
splat in position. This alone necessitated a VWADD_W_VL combine function
which made forming vwadd.vv in two stages an easy choice.

I've left out trying hard to form vwadd.wx instructions for now. It would
only save an extend in the scalar domain which isn't as interesting.

Might need to review the test coverage a bit. Most of the vwadd.wv
instructions are coming from vXi64 tests on rv64. The tests were
copy pasted from the existing multiply tests.

Reviewed By: rogfer01

Differential Revision: https://reviews.llvm.org/D117954