[DAGCombiner] reduce extract subvector of concat

If we are extracting a chunk of a vector that's a fraction of an
operand of the concatenated vector operand, we can extract directly
from one of those original operands.

This is another suggestion from PR42024:
https://bugs.llvm.org/show_bug.cgi?id=42024#c2

But I'm not sure yet if it will make any difference on those patterns.
It seems to help a few existing AVX512 tests though.

Differential Revision: https://reviews.llvm.org/D72361