This patch gives SelectionDAG::isSplatValue the ability to look
through ABS nodes, which allows the combine introduced in D83602 to
scalarize ABS operations when only a single lane is needed.
WebAssembly does not support ABS natively as a scalar operation, but
we still want it to be scalarized and expanded, so this patch also
adds a custom combine in the WebAssembly backend that scalarizes ABS
specifically when it is a splat and only one lane is needed. This is
useful when scalarizing WebAssembly shift values, which often contain
ABS operations in Halide output.
Depends on D83602.