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/meta/src/shared/instructions.rs

@@ -4491,24 +4491,5 @@ pub(crate) fn define(
         .other_side_effects(true),
     );
 
-    let Offset = &Operand::new("Offset", &imm.offset32).with_doc("Byte offset from base address");
-    let a = &Operand::new("a", TxN);
-
-    ig.push(
-        Inst::new(
-            "load_splat",
-            r#"
-        Load an element from memory at ``p + Offset`` and return a vector
-        whose lanes are all set to that element.
-
-        This is equivalent to ``load`` followed by ``splat``.
-        "#,
-            &formats.load,
-        )
-        .operands_in(vec![MemFlags, p, Offset])
-        .operands_out(vec![a])
-        .can_load(true),
-    );
-
     ig.build()
 }