machinst x64: revamp integer immediate emission;

In particular:

- try to optimize the integer emission into a 32-bit emission, when the
high bits are all zero, and stop relying on the caller of `imm_r` to
ensure this.
- rename `Inst::imm_r`/`Inst::Imm_R` to `Inst::imm`/`Inst::Imm`.
- generate a sign-extending mov 32-bit immediate to 64-bits, whenever
possible.
- fix a few places where the previous commit did introduce the
generation of zero-constants with xor, when calling `put_input_to_reg`,
thus clobbering the flags before they were read.

cranelift/codegen/src/isa/x64/inst/args.rs

@@ -1001,6 +1001,14 @@ pub enum OperandSize {
 }
 
 impl OperandSize {
+    pub(crate) fn from_bytes(num_bytes: u32) -> Self {
+        match num_bytes {
+            1 | 2 | 4 => OperandSize::Size32,
+            8 => OperandSize::Size64,
+            _ => unreachable!(),
+        }
+    }
+
     pub(crate) fn to_bytes(&self) -> u8 {
         match self {
             Self::Size32 => 4,