Define HIR more specifically

IR is a foreign acronym to me, so having it fully expressed in the beginning as Intermediate Representation helps me comprehend the subject.

src/high-level-overview.md

@@ -104,8 +104,8 @@ take:
       nodes, and hence may strip things out of the AST as well.
 3. **Lowering to HIR**
     - Once name resolution completes, we convert the AST into the HIR,
-      or "high-level IR". The HIR is defined in `src/librustc/hir/`;
-      that module also includes the lowering code.
+      or "[high-level intermediate representation]". The HIR is defined in
+      `src/librustc/hir/`; that module also includes the lowering code.
     - The HIR is a lightly desugared variant of the AST. It is more processed
       than the AST and more suitable for the analyses that follow.
       It is **not** required to match the syntax of the Rust language.
@@ -138,3 +138,4 @@ take:
 
 
 [query model]: query.html
+[high-level intermediate representation]: hir.html

src/hir.md

@@ -1,10 +1,10 @@
 # The HIR
 
-The HIR – "High-level IR" – is the primary IR used in most of rustc. It is a
-compiler-friendly representation of the abstract syntax tree (AST) that is
-generated after parsing, macro expansion, and name resolution.
-Many parts of HIR resemble Rust surface syntax quite closely, with the
-exception that some of Rust's expression forms have been desugared away. For
+The HIR – "High-Level Intermediate Representation" – is the primary IR used in
+most of rustc. It is a compiler-friendly representation of the abstract syntax
+tree (AST) that is generated after parsing, macro expansion, and name
+resolution. Many parts of HIR resemble Rust surface syntax quite closely, with
+the exception that some of Rust's expression forms have been desugared away. For
 example, `for` loops are converted into a `loop` and do not appear in the HIR.
 This makes HIR more amenable to analysis than a normal AST.