[Bug tree-optimization/126644] New: `a << CST >> CST` can be simplified into a if upper CST bits are not set

[pinskia at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126644

            Bug ID: 126644
           Summary: `a << CST >> CST` can be simplified into a if upper
                    CST bits are not set
           Product: gcc
           Version: 17.0
            Status: UNCONFIRMED
          Keywords: easyhack, missed-optimization, TREE
          Severity: enhancement
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: pinskia at gcc dot gnu.org
  Target Milestone: ---

Take:
```
int h0 (int x, int y, unsigned short *a)
{
    return *a << 3 >> 3;
}
```

This should just be simplified into *a on the tree level.

Currently combine does it:
```
Trying 9 -> 10:
    9: r110:SI=r109:SI<<0x3
      REG_DEAD r109:SI
   10: r108:SI=r110:SI>>0x3
      REG_DEAD r110:SI
Successfully matched this instruction:
(set (reg:SI 108 [ _6 ])
    (reg:SI 109 [ _2 ]))
```
It does via the non-zero bits. A similar thing can be done for match.

Noticed while reviewing
https://gcc.gnu.org/pipermail/gcc-patches/2026-August/726305.html .