[X86] Remove extra getOperand(0) call from recently introduced store(extract_element...

[X86] Remove extra getOperand(0) call from recently introduced store(extract_element(vtrunc)) to truncated store combine.

The IsExtractedElement already called getOperand(0) so Extract
here is the source vector. We shouldn't call getOperand(0). This
worked for the original test cases because the result was a
bitcast so the getOperand(0) accidently peeked through the bitcast
which is what we wanted.

In the failing case here, the operand turns out to be undef so
the getOperand(0) asserts because undef has no operands.

Fixes https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=25184

Differential Revision: https://reviews.llvm.org/D86428