R600: Do not fold modifier/litterals in vector inst

This fixes a couple of regressions on (probably not just) cayman

NOTE: This is a candidate for the Mesa stable branch.

Reviewed-by: Tom Stellard <thomas.stellard at amd.com>
R600: Fold zero/one in export instructions

Reviewed-by: Tom Stellard <thomas.stellard at amd.com>
R600: Export instructions are no longer terminator

This allows MachineInstScheduler to reorder them, and thus make scheduling more
efficient.

Reviewed-by: Tom Stellard <thomas.stellard at amd.com>