Fix warnings produced by g++ on Windows.

Scraped non-pragma related warnings from a recent trybot run:

wget -O - 'http://build.chromium.org/p/tryserver.nacl/builders/nacl-toolchain-win7-pnacl-x86_64/builds/3221/steps/llvm_i686_w64_mingw32%20%28build%29/logs/stdio' | grep subzero | grep warning | grep -v pragma

BUG= none
R=ascull@google.com, jpp@chromium.org

Review URL: https://codereview.chromium.org/1315193020.
diff --git a/src/IceTargetLoweringARM32.cpp b/src/IceTargetLoweringARM32.cpp
index 2be2497..423572c 100644
--- a/src/IceTargetLoweringARM32.cpp
+++ b/src/IceTargetLoweringARM32.cpp
@@ -2259,7 +2259,7 @@
 }
 
 void TargetARM32::lowerIntrinsicCall(const InstIntrinsicCall *Instr) {
-  switch (Intrinsics::IntrinsicID ID = Instr->getIntrinsicInfo().ID) {
+  switch (Instr->getIntrinsicInfo().ID) {
   case Intrinsics::AtomicCmpxchg: {
     UnimplementedError(Func->getContext()->getFlags());
     return;