[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/WebAssemblyISelLowering.cpp b/llvm/lib/Target/WebAssembly/WebAssemblyISelLowering.cpp
index 299009fa..d0b3ad3 100644
--- a/llvm/lib/Target/WebAssembly/WebAssemblyISelLowering.cpp
+++ b/llvm/lib/Target/WebAssembly/WebAssemblyISelLowering.cpp
@@ -117,8 +117,7 @@
   // As a special case, these operators use the type to mean the type to
   // sign-extend from.
   setOperationAction(ISD::SIGN_EXTEND_INREG, MVT::i1, Expand);
-  if (!Subtarget->hasAtomics()) {
-    // The Atomics feature includes signext intructions.
+  if (!Subtarget->hasSignExt()) {
     for (auto T : {MVT::i8, MVT::i16, MVT::i32})
       setOperationAction(ISD::SIGN_EXTEND_INREG, T, Expand);
   }