[Arm] Remove constraint strings from define_expand constructs in the back end

[Arm] Remove constraint strings from define_expand constructs in the back end

A number of Arm define_expand patterns have specified constraints for
their operands. But the constraint strings are ignored at expand time
and are therefore redundant/useless. We now avoid specifying constraints
in new define_expands, but we should clean up the existing define_expand
definitions.

2019-06-28  Dennis Zhang  <dennis.zhang@arm.com>

        * config/arm/arm.md: Remove redundant constraints from
        define_expand but leave reload_inm and reload_outm patterns
        untouched since they need special constraints to work.
        * config/arm/arm-fixed.md: Remove redundant constraints from
        define_expand.
        * config/arm/iwmmxt.md: Likewise.
        * config/arm/neon.md: Likewise.
        * config/arm/sync.md: Likewise.
        * config/arm/thumb1.md: Likewise.
        * config/arm/vec-common.md: Likewise.

From-SVN: r272779