Subzero: Use a proper RegNumT type instead of int32_t/SizeT.

src/IceRegAlloc.h

Index: src/IceRegAlloc.h
diff --git a/src/IceRegAlloc.h b/src/IceRegAlloc.h
index 01c1f3c38d4635d38a5e1cc65ea81fa70492d903..299d709ddc8b85c0a7227723768d490e6b566d12 100644
--- a/src/IceRegAlloc.h
+++ b/src/IceRegAlloc.h
@@ -58,7 +58,7 @@ private:
     IterationState() = default;
     Variable *Cur = nullptr;
     Variable *Prefer = nullptr;
-    int32_t PreferReg = Variable::NoRegister;
+    RegNumT PreferReg = RegNumT::NoRegister;
     bool AllowOverlap = false;
     llvm::SmallBitVector RegMask;
     llvm::SmallBitVector RegMaskUnfiltered;