Remove unused variable introduced in r332336

The unused variable caused a compilation warning:

../lib/Target/X86/X86ISelLowering.cpp:34614:17: error: unused variable 'SMax' [-Werror,-Wunused-variable]
    if (SDValue SMax = MatchMinMax(SMin, ISD::SMAX, C1))
                ^
1 error generated.

llvm-svn: 332431