Add egraph cprop optimizations for splat (#6148)

This commit adds constant-propagation optimizations for
`splat`-of-constant to produce a `vconst` node. This should help later
hoisting these constants out of loops if it shows up in wasm.

cranelift/codegen/src/opts.rs

@@ -4,7 +4,7 @@ use crate::egraph::{NewOrExistingInst, OptimizeCtx};
 use crate::ir::condcodes;
 pub use crate::ir::condcodes::{FloatCC, IntCC};
 use crate::ir::dfg::ValueDef;
-pub use crate::ir::immediates::{Ieee32, Ieee64, Imm64, Offset32, Uimm32, Uimm64, Uimm8};
+pub use crate::ir::immediates::{Ieee32, Ieee64, Imm64, Offset32, Uimm32, Uimm64, Uimm8, V128Imm};
 pub use crate::ir::types::*;
 pub use crate::ir::{
     dynamic_to_fixed, AtomicRmwOp, Block, BlockCall, Constant, DataFlowGraph, DynamicStackSlot,
@@ -128,4 +128,11 @@ impl<'a, 'b, 'c> generated_code::Context for IsleContext<'a, 'b, 'c> {
         self.ctx.stats.subsume += 1;
         value
     }
+
+    fn splat64(&mut self, val: u64) -> Constant {
+        let val = u128::from(val);
+        let val = val | (val << 64);
+        let imm = V128Imm(val.to_le_bytes());
+        self.ctx.func.dfg.constants.insert(imm.into())
+    }
 }