[llvm] [SDPatternMatch] revert SExt changes in #90762 (PR #95415)

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

Cleanup the PR title and summary. Title should be something like "[SDPatternMatch] Only match ISD::SIGN_EXTEND in m_SExt" and then refer to #90762 in the summary and explain why we need this.

https://github.com/llvm/llvm-project/pull/95415