[Bug target/89954] missed optimization for signed extension for x86-64

[ubizjak at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89954

--- Comment #7 from Uroš Bizjak <ubizjak at gmail dot com> ---
Created attachment 51496
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=51496&action=edit
Prototype patch