[WebAssembly] Rename except_ref type to exnref

Summary:
We agreed to rename `except_ref` to `exnref` for consistency with other
reference types in
https://github.com/WebAssembly/exception-handling/issues/79. This also
renames WebAssemblyInstrExceptRef.td to WebAssemblyInstrRef.td in order
to use the file for other reference types in future.

Reviewers: dschuff

Subscribers: sbc100, jgravelle-google, hiraditya, sunfish, jfb, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D64703

llvm-svn: 366145
diff --git a/llvm/lib/Target/WebAssembly/WebAssemblyRegStackify.cpp b/llvm/lib/Target/WebAssembly/WebAssemblyRegStackify.cpp
index 31ba6f0..a120a64 100644
--- a/llvm/lib/Target/WebAssembly/WebAssemblyRegStackify.cpp
+++ b/llvm/lib/Target/WebAssembly/WebAssemblyRegStackify.cpp
@@ -834,9 +834,9 @@
         // entering blocks, which is a part of multi-value proposal.
         //
         // Once we support live-in values of wasm blocks, this can be:
-        // catch                           ; push except_ref value onto stack
-        // block except_ref -> i32
-        // br_on_exn $__cpp_exception      ; pop the except_ref value
+        // catch                           ; push exnref value onto stack
+        // block exnref -> i32
+        // br_on_exn $__cpp_exception      ; pop the exnref value
         // end_block
         //
         // But because we don't support it yet, the catch instruction's dst