Use u32 in Type API (#4280)

Move from passing and returning u8 and u16 values to u32 in many of
the functions. This removes a number of type conversions and gives
a small compilation time speedup, around ~0.7% on my aarch64 machine.

Copyright (c) 2022, Arm Limited.

cranelift/codegen/src/souper_harvest.rs

@@ -535,7 +535,9 @@ fn souper_type_of(dfg: &ir::DataFlowGraph, val: ir::Value) -> Option<ast::Type>
     let ty = dfg.value_type(val);
     assert!(ty.is_int() || ty.is_bool());
     assert_eq!(ty.lane_count(), 1);
-    Some(ast::Type { width: ty.bits() })
+    Some(ast::Type {
+        width: ty.bits().try_into().unwrap(),
+    })
 }
 
 #[derive(Debug)]