[machinst x64]: calculate extension modes in one place

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

@@ -760,6 +760,19 @@ pub enum ExtMode {
 }
 
 impl ExtMode {
+    /// Calculate the `ExtMode` from passed bit lengths of the from/to types.
+    pub(crate) fn new(from_bits: u16, to_bits: u16) -> Option<ExtMode> {
+        match (from_bits, to_bits) {
+            (1, 8) | (1, 16) | (1, 32) | (8, 16) | (8, 32) => Some(ExtMode::BL),
+            (1, 64) | (8, 64) => Some(ExtMode::BQ),
+            (16, 32) => Some(ExtMode::WL),
+            (16, 64) => Some(ExtMode::WQ),
+            (32, 64) => Some(ExtMode::LQ),
+            _ => None,
+        }
+    }
+
+    /// Return the source register size in bytes.
     pub(crate) fn src_size(&self) -> u8 {
         match self {
             ExtMode::BL | ExtMode::BQ => 1,
@@ -767,6 +780,8 @@ impl ExtMode {
             ExtMode::LQ => 4,
         }
     }
+
+    /// Return the destination register size in bytes.
     pub(crate) fn dst_size(&self) -> u8 {
         match self {
             ExtMode::BL | ExtMode::WL => 4,