[ARM] GlobalISel: Fix oversight in G_FCMP legalization

We used to forget to erase the original instruction when replacing a
G_FCMP true/false. Fix this bug and make sure the tests check for it.

llvm-svn: 307639
diff --git a/llvm/lib/Target/ARM/ARMLegalizerInfo.cpp b/llvm/lib/Target/ARM/ARMLegalizerInfo.cpp
index 6b08248..f23e625 100644
--- a/llvm/lib/Target/ARM/ARMLegalizerInfo.cpp
+++ b/llvm/lib/Target/ARM/ARMLegalizerInfo.cpp
@@ -295,6 +295,7 @@
              "Predicate needs libcalls, but none specified");
       MIRBuilder.buildConstant(OriginalResult,
                                Predicate == CmpInst::FCMP_TRUE ? 1 : 0);
+      MI.eraseFromParent();
       return true;
     }