[naga spv-out] Factor out wrapped divide/module generation.

Move the code to generate the definition of an overflow-safe
divide/modulo SPIR-V function into its own Rust function, to reduce
indentation and clarify influences. This commit isn't intended to
cause any change in behavior.

naga/src/back/spv/mod.rs

@@ -303,6 +303,14 @@ impl NumericType {
         }
     }
 
+    const fn scalar(self) -> crate::Scalar {
+        match self {
+            NumericType::Scalar(scalar)
+            | NumericType::Vector { scalar, .. }
+            | NumericType::Matrix { scalar, .. } => scalar,
+        }
+    }
+
     const fn with_scalar(self, scalar: crate::Scalar) -> Self {
         match self {
             NumericType::Scalar(_) => NumericType::Scalar(scalar),