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/ir/immediates.rs

@@ -76,7 +76,7 @@ impl Imm64 {
 
     /// Sign extend this immediate as if it were a signed integer of the given
     /// power-of-two width.
-    pub fn sign_extend_from_width(&mut self, bit_width: u16) {
+    pub fn sign_extend_from_width(&mut self, bit_width: u32) {
         debug_assert!(bit_width.is_power_of_two());
 
         if bit_width >= 64 {