Optimized instruction sequence for sitofp operation on X86-32

Optimized sitofp i64 %x to double. The current sequence

movl %ecx, 8(%esp) 
movl %edx, 12(%esp) 
fildll 8(%esp)

is replaced with:

movd %ecx, %xmm0 
movd %edx, %xmm1 
punpckldq %xmm1, %xmm0 
movq %xmm0, 8(%esp)

Differential Revision: http://reviews.llvm.org/D15946

llvm-svn: 257285