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/codegen/src/isa/aarch64/lower_inst.rs

@@ -1219,6 +1219,7 @@ pub(crate) fn lower_insn_to_regs<C: LowerCtx<I = Inst>>(
             }
         }
 
+        /*
         Opcode::LoadSplat => {
             let off = ctx.data(insn).load_store_offset().unwrap();
             let ty = ty.unwrap();
@@ -1234,6 +1235,7 @@ pub(crate) fn lower_insn_to_regs<C: LowerCtx<I = Inst>>(
                 size,
             });
         }
+        */
 
         Opcode::Store
         | Opcode::Istore8