Reapply "[X86] Remove Redundant memset Calls"

This reverts commit acb798e.

It turns out the memset calls were papering over the fact that the
arrays being used were not initialized rather than papering over a
valgrind issue. Move the initialization to the actual member to keep
things simpler and to be more consistent with the rest of LLVM.

Author: boomanaiden154

llvm/lib/Target/X86/X86FloatingPoint.cpp

@@ -58,12 +58,7 @@ namespace {
 
   struct FPS : public MachineFunctionPass {
     static char 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));
-    }
+    FPS() : MachineFunctionPass(ID) {}
 
     void getAnalysisUsage(AnalysisUsage &AU) const override {
       AU.setPreservesCFG();
@@ -148,7 +143,7 @@ namespace {
     // to model that exactly. Usually, each live register corresponds to an
     // FP<n> register, but when dealing with calls, returns, and inline
     // assembly, it is sometimes necessary to have live scratch registers.
-    unsigned Stack[8];          // FP<n> Registers in each stack slot...
+    unsigned Stack[8] = {};     // FP<n> Registers in each stack slot...
     unsigned StackTop = 0;      // The current top of the FP stack.
 
     enum {
@@ -159,7 +154,7 @@ namespace {
     // The first entries correspond to FP0-FP6, the rest are scratch registers
     // used when we need slightly different live registers than what the
     // register allocator thinks.
-    unsigned RegMap[NumFPRegs];
+    unsigned RegMap[NumFPRegs] = {};
 
     // Set up our stack model to match the incoming registers to MBB.
     void setupBlockStack();