Add a DynRex recipe type for x86, decreasing the number of recipes (#1298)

This patch adds a third mode for templates: REX inference is requestable
at template instantiation time. This reduces the number of recipes
by removing rex()/nonrex() redundancy for many instructions.

cranelift/codegen/meta/src/cdsl/recipes.rs

@@ -172,7 +172,7 @@ pub(crate) struct EncodingRecipeBuilder {
     pub base_size: u64,
     pub operands_in: Option<Vec<OperandConstraint>>,
     pub operands_out: Option<Vec<OperandConstraint>>,
-    compute_size: Option<&'static str>,
+    pub compute_size: Option<&'static str>,
     pub branch_range: Option<BranchRange>,
     pub emit: Option<String>,
     clobbers_flags: Option<bool>,