[X86] Smaller code for materializing 32-bit 1 and -1 constants

"movl $-1, %eax" is 5 bytes, "xorl %eax, %eax; decl %eax" is 3 bytes.
This commit makes LLVM use the latter when optimizing for size.

Differential Revision: http://reviews.llvm.org/D14971

llvm-svn: 255656