Remove LoadSplat opcode, in preparation for pattern-matching Load+Splat.

This was added as an incremental step to improve AArch64 code quality in
PR #2278. At the time, we did not have a way to pattern-match the load +
splat opcode sequence that the relevant Wasm opcodes lowered to.
However, now with PR #2366, we can merge effectful instructions such as
loads into other ops, and so we can do this pattern matching directly.
The pattern-matching update will come in a subsequent commit.

cranelift/interpreter/src/step.rs

@@ -545,7 +545,6 @@ where
         Opcode::Shuffle => unimplemented!("Shuffle"),
         Opcode::Swizzle => unimplemented!("Swizzle"),
         Opcode::Splat => unimplemented!("Splat"),
-        Opcode::LoadSplat => unimplemented!("LoadSplat"),
         Opcode::Insertlane => unimplemented!("Insertlane"),
         Opcode::Extractlane => unimplemented!("Extractlane"),
         Opcode::VhighBits => unimplemented!("VhighBits"),