Adds a new ISD node to replicate a scalar value across all elements of a vector. This is needed for scalable vectors, since BUILD_VECTOR cannot be used.
At present I only use this node for scalable vectors; although the default expand works and generates a build vector with the appropriate value, some backends expect a different canonical form in SDag for a splat. I suspect the code to generate those shuffles will need to be replicated in the expansion (or possibly moved to a shared function) before this can be used by default for all backends.
I've only added legalization for i8/i16/i32/i64 types in AArch64 for now.