Revert "[X86] Remove Redundant memset Calls"

This reverts commit 4b805e1.

It turns out the original commit was wrong and these were not just
quieting valgrind down, but actually solving an issue. We now get MSan
failures. Reverting to have some time to investigate.

https://lab.llvm.org/buildbot/#/builders/164/builds/15562

Author: boomanaiden154

llvm/lib/Target/X86/X86FloatingPoint.cpp

@@ -58,7 +58,12 @@ namespace {
 
   struct FPS : public MachineFunctionPass {
     static char ID;
-    FPS() : MachineFunctionPass(ID) {}
+    FPS() : MachineFunctionPass(ID) {
+      // This is really only to keep valgrind quiet.
+      // The logic in isLive() is too much for it.
+      memset(Stack, 0, sizeof(Stack));
+      memset(RegMap, 0, sizeof(RegMap));
+    }
 
     void getAnalysisUsage(AnalysisUsage &AU) const override {
       AU.setPreservesCFG();