[CostModel] Fixed isExtractSubvectorMask for undef index off end

ShuffleVectorInst::isExtractSubvectorMask, introduced in
  [CostModel] Add SK_ExtractSubvector handling to getInstructionThroughput (PR39368)

erroneously thought that
%340 = shufflevector <4 x float> %339, <4 x float> undef, <3 x i32> <i32 2, i32 3, i32 undef>

is a subvector extract, even though it goes off the end of the parent
vector with the undef index. That then caused an assert in
BasicTTIImplBase::getExtractSubvectorOverhead.

This commit fixes that, by not considering the above a subvector
extract.

Differential Revision: https://reviews.llvm.org/D70005

Change-Id: I87b8b00b24bef19ffc9a1b82ef4eca3b8a246eaf