[AMDGPU] Make uniform-work-group-size a valueless attribute (#183925)

The "uniform-work-group-size" function attribute previously took a
string value of "true" or "false". Since presence alone can convey the
"true" semantics and absence can convey "false", the value is
unnecessary.

This patch converts it to a valueless string attribute: presence
indicates true, absence indicates false. For backward compatibility,
auto-upgrade logic is added in both UpgradeAttributes (bitcode) and
UpgradeFunctionAttributes: if the old value is "true", the attribute is
kept without a value; if "false", the attribute is removed.
GitOrigin-RevId: f05d2e8a39987b8d87e0a7d6ef887749b5b1700e