[X86] Do not generate 64-bit pops of 32-bit GPRs.

When trying emit a stack adjustments using pops, frame lowering selects an
arbitrary free GPR. It should always select one from an appropriate class...
This fixes PR24649.

Patch by: amjad.aboud@intel.com
Differential Revision: http://reviews.llvm.org/D12609

llvm-svn: 247785
diff --git a/llvm/lib/Target/X86/X86FrameLowering.cpp b/llvm/lib/Target/X86/X86FrameLowering.cpp
index edc4114..c9715f1 100644
--- a/llvm/lib/Target/X86/X86FrameLowering.cpp
+++ b/llvm/lib/Target/X86/X86FrameLowering.cpp
@@ -1935,6 +1935,9 @@
 bool X86FrameLowering::adjustStackWithPops(MachineBasicBlock &MBB,
     MachineBasicBlock::iterator MBBI, DebugLoc DL, int Offset) const {
 
+  if (Offset <= 0)
+    return false;
+
   if (Offset % SlotSize)
     return false;
 
@@ -1955,9 +1958,11 @@
   unsigned FoundRegs = 0;
 
   auto RegMask = Prev->getOperand(1);
-  
-  // Try to find up to NumPops free registers. 
-  for (auto Candidate : X86::GR32_NOREX_NOSPRegClass) {
+
+  auto &RegClass =
+      Is64Bit ? X86::GR64_NOREX_NOSPRegClass : X86::GR32_NOREX_NOSPRegClass;
+  // Try to find up to NumPops free registers.
+  for (auto Candidate : RegClass) {
 
     // Poor man's liveness:
     // Since we're immediately after a call, any register that is clobbered