More i8 legalizations (#1253)

* Legalize stack_{load,store}.i8, fixes #433
* Legalize select.i8, cc: #466
* Legalize brz.i8 and brnz.i8, cc: #1117

cranelift/codegen/meta/src/shared/legalize.rs

@@ -139,6 +139,7 @@ pub(crate) fn define(insts: &InstructionGroup, imm: &Immediates) -> TransformGro
     expand.custom_legalize(trapnz, "expand_cond_trap");
     expand.custom_legalize(br_table, "expand_br_table");
     expand.custom_legalize(select, "expand_select");
+    widen.custom_legalize(select, "expand_select"); // small ints
 
     // Custom expansions for floating point constants.
     // These expansions require bit-casting or creating constant pool entries.
@@ -149,6 +150,10 @@ pub(crate) fn define(insts: &InstructionGroup, imm: &Immediates) -> TransformGro
     expand.custom_legalize(insts.by_name("stack_load"), "expand_stack_load");
     expand.custom_legalize(insts.by_name("stack_store"), "expand_stack_store");
 
+    // Custom expansions for small stack memory acccess.
+    widen.custom_legalize(insts.by_name("stack_load"), "expand_stack_load");
+    widen.custom_legalize(insts.by_name("stack_store"), "expand_stack_store");
+
     // List of variables to reuse in patterns.
     let x = var("x");
     let y = var("y");
@@ -612,6 +617,18 @@ pub(crate) fn define(insts: &InstructionGroup, imm: &Immediates) -> TransformGro
         }
     }
 
+    for &ty in &[I8, I16] {
+        widen.legalize(
+            def!(brz.ty(x, ebb, vararg)),
+            vec![def!(a = uextend.I32(x)), def!(brz(a, ebb, vararg))],
+        );
+
+        widen.legalize(
+            def!(brnz.ty(x, ebb, vararg)),
+            vec![def!(a = uextend.I32(x)), def!(brnz(a, ebb, vararg))],
+        );
+    }
+
     // Expand integer operations with carry for RISC architectures that don't have
     // the flags.
     let intcc_ult = Literal::enumerator_for(&imm.intcc, "ult");