[Driver] Add support for -msve-vector-bits=scalable.

No real action is taken for a value of scalable but it provides a
route to disable an earlier specification and is effectively its
default value when omitted.

Patch also removes an "unused variable" warning.

Differential Revision: https://reviews.llvm.org/D84021
diff --git a/clang/lib/Driver/ToolChains/Arch/AArch64.cpp b/clang/lib/Driver/ToolChains/Arch/AArch64.cpp
index 428b72a..43959f5 100644
--- a/clang/lib/Driver/ToolChains/Arch/AArch64.cpp
+++ b/clang/lib/Driver/ToolChains/Arch/AArch64.cpp
@@ -370,8 +370,8 @@
     V8_6Pos = Features.insert(std::next(V8_6Pos), {"+i8mm", "+bf16"});
 
   bool HasSve = llvm::is_contained(Features, "+sve");
-  // -msve_vector_bits=<bits> flag is valid only if SVE is enabled.
-  if (Arg *A = Args.getLastArg(options::OPT_msve_vector_bits_EQ))
+  // -msve-vector-bits=<bits> flag is valid only if SVE is enabled.
+  if (Args.hasArg(options::OPT_msve_vector_bits_EQ))
     if (!HasSve)
       D.Diag(diag::err_drv_invalid_sve_vector_bits);