[DAGCombine] Combine signext_inreg of extract-extend

The outer signext_inreg is redundant in the following:

  Fold (signext_inreg (extract_subvector (zext|anyext|sext iN_value to _) _) from iN)
       -> (extract_subvector (signext iN_value to iM))

Tests are precommitted and clone those by analogy from the AND case in
the same file. Add a negative test to check extension width is handled
correctly.

This patch supersedes D130700.

Differential Revision: https://reviews.llvm.org/D131503