[SelectionDAGBuilder] Use APInt::isZero instead of comparing...

[SelectionDAGBuilder] Use APInt::isZero instead of comparing APInt::getZExtValue to 0 in a place where we can't be sure contents of the APInt fit in a uint64_t.

This is used on an extract vector element index which is most cases is going to be an i32 or i64 and the element will be a valid element number. But it is possible to construct IR with a larger type and large out of range value.

llvm-svn: 337652