Add a regs_overlap function to the isa module.

Test it with the arm32 register banks which have the most interesting
properties. Most other registers have a single register unit.

lib/cretonne/src/isa/registers.rs

@@ -188,6 +188,16 @@ impl fmt::Display for RegClassIndex {
     }
 }
 
+/// Test of two registers overlap.
+///
+/// A register is identified as a `(RegClass, RegUnit)` pair. The register class is needed to
+/// determine the width (in regunits) of the register.
+pub fn regs_overlap(rc1: RegClass, reg1: RegUnit, rc2: RegClass, reg2: RegUnit) -> bool {
+    let end1 = reg1 + rc1.width as RegUnit;
+    let end2 = reg2 + rc2.width as RegUnit;
+    !(end1 <= reg2 || end2 <= reg1)
+}
+
 /// Information about the registers in an ISA.
 ///
 /// The `RegUnit` data structure collects all relevant static information about the registers in an