Remove the widening_pairwise_dot_product_s clif instruction (#5889)

This was added for the wasm SIMD proposal but I've been poking around at
this recently and the instruction can instead be represented by its
component parts with the same semantics I believe. This commit removes
the instruction and instead represents it with the existing
`iadd_pairwise` instruction (among others) and updates backends to with
new pattern matches to have the same codegen as before.

This interestingly entirely removed the codegen rule with no replacement
on the AArch64 backend as the existing rules all existed to produce the
same codegen.

cranelift/wasm/src/code_translator.rs

@@ -2059,7 +2059,13 @@ pub fn translate_operator<FE: FuncEnvironment + ?Sized>(
         }
         Operator::I32x4DotI16x8S => {
             let (a, b) = pop2_with_bitcast(state, I16X8, builder);
-            state.push1(builder.ins().widening_pairwise_dot_product_s(a, b));
+            let alow = builder.ins().swiden_low(a);
+            let blow = builder.ins().swiden_low(b);
+            let low = builder.ins().imul(alow, blow);
+            let ahigh = builder.ins().swiden_high(a);
+            let bhigh = builder.ins().swiden_high(b);
+            let high = builder.ins().imul(ahigh, bhigh);
+            state.push1(builder.ins().iadd_pairwise(low, high));
         }
         Operator::I8x16Popcnt => {
             let arg = pop1_with_bitcast(state, type_of(op), builder);