[WebAssembly] Make sign-extension opcodes a distinct feature.

Sign-extension opcodes have been split into a separate proposal from
the main threads proposal, so switch them to their own target
feature. See:

https://github.com/WebAssembly/sign-extension-ops

llvm-svn: 322966
diff --git a/llvm/lib/Target/WebAssembly/WebAssemblySubtarget.cpp b/llvm/lib/Target/WebAssembly/WebAssemblySubtarget.cpp
index 9e122a5..78602a3 100644
--- a/llvm/lib/Target/WebAssembly/WebAssemblySubtarget.cpp
+++ b/llvm/lib/Target/WebAssembly/WebAssemblySubtarget.cpp
@@ -41,8 +41,8 @@
                                            const std::string &FS,
                                            const TargetMachine &TM)
     : WebAssemblyGenSubtargetInfo(TT, CPU, FS), HasSIMD128(false),
-      HasAtomics(false), HasNontrappingFPToInt(false), CPUString(CPU),
-      TargetTriple(TT), FrameLowering(),
+      HasAtomics(false), HasNontrappingFPToInt(false), HasSignExt(false),
+      CPUString(CPU), TargetTriple(TT), FrameLowering(),
       InstrInfo(initializeSubtargetDependencies(FS)), TSInfo(),
       TLInfo(TM, *this) {}