[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