[X86] Replace duplicate function call with variable. NFC

Change from:
if (N->getOperand(0).getValueType() == MVT::v8i32 ||
    N->getOperand(0).getValueType() == MVT::v8f32)

to:
EVT OpVT = N->getOperand(0).getValueType();
if (OpVT == MVT::v8i32 || OpVT == MVT::v8f32)

Change-Id: I5a105f8710b73a828e6cfcd55fac2eae6153ce25
llvm-svn: 317464